Cheesy Summer Squash and Zucchini Casserole tastes rich, creamy, and just a little bit smoky, with tender veggies tucked under a bubbling golden cheese crust. It works perfectly for busy families, potlucks, or anyone who wants a cozy veggie-forward dinner on the table in about 50–60 minutes. I tested this version after my neighbor dropped off a mountain of zucchini, and my kids actually asked for seconds, which almost never happens with vegetables.

Why Cheesy Summer Squash and Zucchini Casserole Is Worth It

This casserole turns simple summer squash and zucchini into something that tastes like comfort food and farmers’ market had a very cheesy baby. The veggies soften into silky layers, the cheese melts into every nook, and the top bakes into a crisp, browned lid that shatters slightly when you scoop.

You can prep it ahead, bake it when you need it, and serve it as a main or a side. It uses basic ingredients, welcomes substitutions, and rescues that pile of zucchini on your counter before it goes sad and floppy.

“This Cheesy Summer Squash and Zucchini Casserole tastes like mac and cheese met a garden, and everyone at the table scraped their plates clean. ★★★★★”

Ingredients You Need

Veggies

3 medium zucchini, thinly sliced into rounds (about 1/4-inch thick)
3 medium yellow summer squash, thinly sliced into rounds
1 medium yellow onion, thinly sliced
2 cloves garlic, minced (jarred minced garlic works in a pinch)

Dairy & Eggs

1 cup whole milk (2% works; avoid skim for best texture)
1/2 cup heavy cream (or use all milk for a lighter version)
2 large eggs
1 1/2 cups shredded sharp cheddar cheese, divided
- I like extra-sharp cheddar from Tillamook or Cabot for big flavor.
1/2 cup shredded mozzarella cheese
1/4 cup grated Parmesan cheese (from a wedge if possible, not the green can)

Pantry Staples & Seasoning

3 tablespoons olive oil or avocado oil
2 tablespoons butter, plus more for greasing the dish
1/2 cup panko breadcrumbs (Italian seasoned works great)
2 tablespoons all-purpose flour (or cornstarch for gluten-free)
1 teaspoon kosher salt, plus more to taste
1/2 teaspoon freshly ground black pepper
1 teaspoon dried Italian seasoning or herbes de Provence
1/2 teaspoon smoked paprika (or sweet paprika if you prefer mild flavor)
1/4 teaspoon red pepper flakes (optional, for a little heat)

Fresh Herbs (Optional but tasty)

2 tablespoons chopped fresh basil or parsley
1 teaspoon fresh thyme leaves

Equipment

9×13-inch baking dish or similar casserole dish
Large skillet or sauté pan
Medium saucepan
Mixing bowls
Whisk
Sharp knife and cutting board
Mandoline slicer (optional, for even veggie slices)

Quick Tips & substitutions

Slice zucchini and squash evenly so they cook at the same rate and stay tender, not mushy.
Salt the sliced veggies lightly and let them sit 10–15 minutes, then pat dry to reduce excess moisture.
Swap cheddar with Gruyère, Monterey Jack, or a blend if you want a different cheese vibe.
Use cornstarch instead of flour to keep the casserole gluten-free.
Skip heavy cream and use all milk for a lighter version; just cook the sauce a bit longer to thicken.
Use pre-shredded cheese when you need speed, but shred from a block if you want the smoothest melt.
Add a handful of cooked chicken or turkey sausage if you want extra protein.
Toss in a cup of cherry tomatoes, halved, for juicy pops of flavor.
Use gluten-free panko or crushed gluten-free crackers on top if you avoid gluten.
Prep the casserole earlier in the day, cover, and chill; add 5–10 minutes to the bake time from cold.

How to Make Cheesy Summer Squash and Zucchini Casserole

Step 1: Prep and season the veggies

Slice zucchini and summer squash into even rounds about 1/4-inch thick. Lay them on a sheet pan or large cutting board, sprinkle with a little salt, and let them sit 10–15 minutes. This step pulls out extra moisture so the casserole bakes creamy instead of watery. Pat the slices dry with paper towels.

Step 2: Sauté veggies and aromatics

Heat 2 tablespoons olive oil in a large skillet over medium heat. Add the sliced onion and cook until it turns soft and lightly golden, about 8–10 minutes, then stir in the garlic and cook 1 minute more. Add the zucchini and summer squash, a pinch of salt and pepper, and cook 5–7 minutes, stirring occasionally, until they soften slightly but still hold their shape. Turn off the heat and set the skillet aside.

Step 3: Make the cheesy sauce

In a medium saucepan, melt 2 tablespoons butter over medium heat. Whisk in the flour and cook 1–2 minutes, whisking constantly, until the mixture looks smooth and slightly foamy. Slowly pour in the milk and cream while you whisk, and keep whisking until the sauce thickens and coats the back of a spoon, about 4–6 minutes. Season with salt, pepper, Italian seasoning, and smoked paprika.

Take the saucepan off the heat. Stir in 1 cup shredded cheddar, mozzarella, and Parmesan until the cheese melts and the sauce turns smooth. Taste and adjust seasoning with more salt or pepper if needed. Let the sauce cool 2–3 minutes, then whisk in the eggs quickly so they blend without scrambling.

Step 4: Assemble the casserole

Grease a 9×13-inch baking dish with butter or oil. Spread half of the sautéed veggie mixture in an even layer in the dish. Pour half of the cheesy sauce over the veggies and spread it gently with a spatula. Add the remaining veggies on top, then pour the rest of the sauce over everything.

Sprinkle the remaining 1/2 cup cheddar evenly over the top. In a small bowl, mix panko breadcrumbs with 1 tablespoon olive oil and a pinch of salt and pepper, then scatter that over the cheese. If you use fresh herbs like thyme, sprinkle a little over the top as well. The casserole should look generously covered but not buried.

Step 5: Bake until bubbly and golden

Heat the oven to 375°F (190°C). Place the casserole on the middle rack and bake 25–35 minutes, until the edges bubble and the top turns deep golden brown. If the top browns too quickly, tent the dish loosely with foil and keep baking until the center sets. The casserole should feel set when you nudge the dish, not sloshy.

Step 6: Rest, garnish, and serve

Let the Cheesy Summer Squash and Zucchini Casserole rest 10–15 minutes before cutting so it sets and slices cleanly. Sprinkle chopped basil or parsley over the top for color and freshness. Cut into squares or scoop with a big spoon, depending on your mood and how hungry everyone looks. Serve warm and watch the cheese stretch like a tiny kitchen magic trick.

Recipe Variations

Gluten-free: Use cornstarch instead of flour in the sauce and gluten-free panko or crushed gluten-free crackers on top.
Vegan: Swap dairy milk and cream with unsweetened oat or almond milk, use vegan butter, and choose a good melting vegan cheese; skip eggs and thicken the sauce a bit more on the stove.
Low carb: Skip the breadcrumbs and add extra Parmesan on top; use a bit more cheese and less milk for a thicker, richer sauce.
Extra protein: Stir in cooked shredded chicken, ground turkey, or crumbled cooked tofu with the veggies.
Herb-forward: Add fresh thyme, oregano, or chives to the sauce and sprinkle extra on top before serving.
Spicy: Increase red pepper flakes and add a pinch of cayenne to the sauce.

Ways to Serve Cheesy Summer Squash and Zucchini Casserole

Serve as a main dish with a crisp green salad and crusty garlic bread.
Pair with grilled chicken, pork chops, or roasted salmon as a hearty side.
Spoon leftovers into a warm tortilla with a little salsa for a quick veggie quesadilla filling.
Serve alongside scrambled eggs or an omelet for a cozy brunch plate.
Pack a square in a lunchbox with some fruit and crunchy veggies on the side.

Storage Success

Let the Cheesy Summer Squash and Zucchini Casserole cool to room temperature, then cover the dish tightly or transfer slices to airtight containers. Store in the fridge for up to 4 days, and reheat in the oven at 350°F until hot, or in the microwave in short bursts so the cheese stays creamy. Freeze individual portions for up to 2 months; wrap them well to prevent freezer burn. Thaw in the fridge overnight, then reheat until the center steams and the top turns melty again.