Squash and Zucchini Casserole with Parmesan Crumb Topping

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This squash and zucchini casserole is a lighter, garden-fresh take on the classic. Yellow squash and zucchini are baked in a creamy custard made with Greek yogurt, eggs and nutty white cheddar, then finished with a crisp, golden Parmesan and panko crumb. It has all the cozy, cheesy appeal of a traditional squash casserole at around 210 calories a serving, and it’s a beautiful way to use up a summer’s worth of squash.

If you’re craving the old-fashioned version with a buttery Ritz topping, head over to my Southern squash casserole. This one is its brighter, herby cousin.

Why You’ll Love This Casserole

• Two kinds of squash. Yellow squash is sweet and tender, and zucchini adds color and a slightly firmer bite.
• Lighter but still creamy. Greek yogurt replaces most of the sour cream and butter, adding protein and a gentle tang.
• Crispy Parmesan crumb. Panko and Parmesan bake into a crunchy, savory topping that stays crisp.
• Roasted, not boiled. Roasting the squash first concentrates the flavor and removes the extra water.
• Vegetarian and versatile. Serve it as a side, or as a light main with a salad and crusty bread.

Ingredients For Squash and Zucchini Casserole

You’ll need a handful of fresh vegetables and a few fridge staples. Full amounts are in the recipe card below.

For the filling

• Yellow squash and zucchini: 1 pound of each, about 3 medium of each, sliced into ¼-inch rounds.
• Onion and garlic: one small onion, thinly sliced, and two garlic cloves.
• Olive oil and salt: for roasting the vegetables.
• Eggs: two large eggs to set the custard.
• Plain Greek yogurt: ½ cup, whole milk or 2%, for creaminess without heaviness.
• White cheddar or Gruyère: 1 cup, shredded. Both melt beautifully and pair well with squash.
• Fresh thyme: or ½ teaspoon dried thyme, plus black pepper.

For the Parmesan crumb

• Panko breadcrumbs: ¾ cup for a light, shatter-crisp crunch.
• Parmesan: about ½ cup, finely grated, split between the filling and topping.
• Melted butter or olive oil: 1 tablespoon to help the crumbs brown.
• Parsley: for a fresh finish.

Equipment

• Two large rimmed baking sheets for roasting
• A 2-quart gratin dish or 11×7-inch baking dish
• A sharp knife or mandoline
• A mixing bowl and whisk

How to Make Squash and Zucchini Casserole

Step 1: Slice the vegetables

Preheat the oven to 425°F (220°C). Trim the squash and zucchini and slice them into even ¼-inch rounds. Thinly slice the onion.

Step 2: Roast to remove the water

Divide the squash, zucchini and onion between two baking sheets, drizzle with olive oil, sprinkle with salt and toss. Spread them in a single layer and roast for 15 to 18 minutes, until tender with a few golden edges. Using two pans keeps the vegetables from steaming. Lower the oven to 375°F (190°C) when they come out.

Step 3: Make the custard

In a large bowl, whisk the eggs, Greek yogurt, minced garlic, thyme and pepper until smooth. Stir in the white cheddar and ¼ cup of the Parmesan.

Step 4: Combine and layer

Let the roasted vegetables cool for about 5 minutes, then fold them gently into the custard. Spread everything in a greased 2-quart baking dish, arranging some of the prettiest slices on top if you like.

Step 5: Add the Parmesan crumb

Mix the panko, remaining Parmesan and melted butter until the crumbs look like damp sand. Scatter them evenly over the casserole.

Step 6: Bake and rest

Bake at 375°F for 25 to 30 minutes, until the custard is set in the center and the topping is golden and crisp. Let it rest for 10 minutes, then sprinkle with chopped parsley and serve.

Tips for the Best Results

• Don’t crowd the pans. Crowded vegetables steam instead of roast and keep all their water.
• Slice evenly. Even slices roast at the same rate, so nothing ends up mushy or raw.
• Use full-fat or 2% yogurt. Nonfat yogurt can make the custard slightly grainy.
• Grate your own Parmesan. Freshly grated cheese browns and melts better than the shelf-stable kind.
• Season boldly. Squash is mild, so a little extra pepper, thyme or a pinch of red pepper flakes goes a long way.

Troubleshooting

It’s watery on the bottom. The vegetables needed a bit more roasting, or the pans were crowded. Roast until the edges start to brown.

The custard looks curdled. Hot vegetables can scramble the eggs. Let them cool for a few minutes before mixing.

The topping isn’t crispy. Make sure the crumbs are coated in butter or oil, and bake on the middle rack. A 1-minute broil at the end helps too.

Variations

• Add tomatoes: layer thin tomato slices on top before the crumbs for a summery tian-style casserole.
• Make it Italian: use mozzarella instead of cheddar, add basil and a spoonful of pesto to the custard.
• Add protein: fold in 2 cups of shredded cooked chicken or crumbled cooked turkey sausage for a one-dish meal.
• Go gluten-free: use gluten-free panko or crushed pork rinds in the topping.
• Extra veggies: add sautéed mushrooms, corn kernels or a diced red bell pepper.

What to Serve With It

This casserole works as a side for almost any protein. It’s great with grilled chicken, roast pork tenderloin, salmon or steak. It pairs especially well with my 3-ingredient baked Caesar chicken or Greek meatballs with orzo. For a vegetarian dinner, serve a generous square with a green salad and crusty bread. If you love a good veggie bake, you’ll also like this roasted vegetable baked ziti.

Make Ahead and Storage

Can I prepare it ahead?

Roast the vegetables up to 2 days ahead and refrigerate them. On the day, mix the custard, assemble and bake, adding about 5 minutes since the vegetables will be cold. Keep the crumb topping separate until baking.

How long do leftovers keep?

Store leftovers in an airtight container in the fridge for up to 4 days. Reheat in a 350°F oven or air fryer for 10 to 15 minutes to re-crisp the topping.

Can I freeze it?

It’s best fresh, since squash softens a lot when frozen. If you do freeze it, wrap well and use within 1 month, then reheat in the oven.

Why Roasting Makes a Better Squash Casserole

The biggest complaint about squash casserole is that it turns watery. Squash and zucchini are mostly water, and if you add them raw or boiled, that water ends up in the bottom of the dish. Roasting solves this in two ways: the dry heat evaporates moisture before the vegetables ever meet the custard, and it caramelizes their natural sugars for a deeper, slightly sweet flavor. It takes about 15 minutes, and it’s the single step that makes this casserole taste special.

Nutrition at a Glance

Each serving has roughly 210 calories, 12 grams of protein and about 11 grams of carbohydrates. The Greek yogurt and eggs add protein, while the squash and zucchini bring fiber, potassium and vitamin C. Compared with a traditional squash casserole made with butter crackers and sour cream, this version has about a third fewer calories and less saturated fat, but it’s still rich, cheesy and satisfying.

If you’re counting carbs, skip the panko and use extra Parmesan mixed with a few tablespoons of almond flour for the topping. It still browns nicely and keeps the casserole low-carb and keto-friendly.

Choosing the Best Cheese

The cheese does a lot of the heavy lifting in a squash casserole, so it’s worth choosing one with plenty of flavor. White cheddar is my go-to because it’s sharp, creamy and melts smoothly. Gruyère is a wonderful alternative, with a nutty, slightly sweet flavor that makes the casserole taste like a French gratin. Fontina is milder and extra melty, and smoked Gouda adds a subtle smoky note that’s especially good in fall.

Whatever you choose, shred it yourself from a block. Pre-shredded cheese is coated with anti-caking powders that keep it from melting into a smooth, creamy custard. The Parmesan should also be freshly grated, since it’s the key to the golden, savory crust on top.

Summer Squash Varieties That Work

Any summer squash works in this recipe, so feel free to use what looks best at the market. Straight-neck and crookneck yellow squash are the most common and have a mild, buttery flavor. Classic green zucchini is slightly firmer. Golden zucchini looks like yellow squash but has the texture of zucchini, and pattypan squash, the little scalloped ones, can be sliced and used the same way. Mixing a few varieties makes the casserole even prettier.

Skip winter squash like butternut or acorn here. They’re much denser, need a longer cook time and won’t give you the same light, custardy texture.

Serving It for the Holidays

Although it’s a summer favorite, this casserole is also a nice change of pace on a holiday table. It’s lighter than most traditional sides, so it balances rich dishes like mashed potatoes, stuffing and gravy. To make it more festive, layer thin slices of yellow squash and zucchini in alternating rings on top before adding the crumb, then garnish with fresh thyme sprigs after baking. You can double the recipe in a 9×13-inch dish; bake it for 35 to 40 minutes.

Frequently Asked Questions

Can I use only zucchini or only yellow squash?

Yes. Use 2 pounds of either one. All zucchini gives a slightly firmer texture and a greener look; all yellow squash is a little sweeter.

Do I need to peel the squash?

No. The skin is thin and tender, and it adds color and nutrients. Just rinse and trim the ends.

Can I use sour cream instead of Greek yogurt?

Absolutely. Sour cream makes it a bit richer. Use the same amount.

Is this casserole vegetarian?

Yes, as long as your Parmesan is made without animal rennet. Many brands now label vegetarian-friendly Parmesan.

How do I know when it’s done?

The center should look set and not jiggle when you gently shake the dish, and the topping should be golden brown. A knife inserted in the middle should come out mostly clean.

Fresh, Cheesy and Easy

With roasted squash, a creamy yogurt custard and a crispy Parmesan crumb, this casserole is proof that lighter comfort food can still be incredibly satisfying. Give it a try this week, then leave a star rating and tell me which cheese you used. Save it to your Pinterest vegetable side dish board for your next farmers’ market haul.

Nutrient	Amount
Calories	210 kcal
Protein	12 g
Carbohydrates	11 g
Fat	13 g
Saturated Fat	6 g
Fiber	2 g
Sugar	4 g
Sodium	410 mg

Nutrition values are estimates calculated from standard ingredient data and will vary with the brands and exact amounts you use.