Cheesy Summer Squash and Zucchini Casserole tastes like creamy, garlicky mac and cheese met late-summer veggies and decided to get baked under a golden crust. It works perfectly for busy weeknights or casual gatherings, since it comes together in about 55 minutes start to finish. I tested this one on my picky siblings in Ohio, and they went quiet at the table, which counts as the highest praise in my family.
Why Choose This Cheesy Summer Squash and Zucchini Casserole
This casserole turns simple summer squash and zucchini into a cheesy, bubbly pan of comfort that still feels light enough for warm weather. It uses basic ingredients, so you skip fancy shopping trips and still pull off something that tastes restaurant-level.
You can serve it as a vegetarian main or as a hearty side with grilled chicken or fish. It also works well for meal prep, since leftovers reheat beautifully and the flavors deepen overnight.
“This Cheesy Summer Squash and Zucchini Casserole tasted rich, cozy, and full of fresh veggie flavor, and my family scraped the pan clean. ★★★★★”
Ingredients You’ll Need
Vegetables
- 2 medium yellow summer squash, thinly sliced into rounds
- 2 medium zucchini, thinly sliced into rounds
- 1 small yellow onion, thinly sliced
- 2 cloves garlic, minced
- 2 tablespoons olive oil or avocado oil
I like to slice the squash and zucchini about 1/4 inch thick so they soften but still hold their shape. You can use all zucchini or all yellow squash if that is what you have on hand. Frozen sliced zucchini works in a pinch, but pat it very dry so it does not water down the casserole.
Cheesy Sauce & Topping
- 3 tablespoons unsalted butter
- 3 tablespoons all-purpose flour
- 1 1/2 cups whole milk (or 2% in a pinch)
- 1/2 cup heavy cream or half-and-half
- 1 teaspoon kosher salt, plus more to taste
- 1/2 teaspoon black pepper
- 1/2 teaspoon garlic powder
- 1/2 teaspoon onion powder
- 1/4 teaspoon smoked paprika (optional, for a subtle smoky note)
- 1 1/2 cups shredded sharp cheddar cheese, divided
- 1/2 cup shredded mozzarella cheese
- 1/4 cup grated Parmesan cheese
Sharp cheddar gives the casserole a bold cheesy flavor, so I reach for a block and shred it myself. Pre-shredded cheese works if you feel short on time, but it melts a little less smoothly because of the added starch. You can swap in Monterey Jack or Colby Jack for a milder flavor, or use all mozzarella for extra stretch.
Crunchy Topping
- 3/4 cup panko breadcrumbs or regular breadcrumbs
- 2 tablespoons grated Parmesan cheese
- 2 tablespoons melted butter or olive oil
- 1 tablespoon chopped fresh parsley or chives (optional)
Panko gives a lighter, crispier topping, while regular breadcrumbs create a more classic crust. Gluten-free breadcrumbs work well here, so you can keep the casserole gluten-free if you also swap the flour for a 1:1 gluten-free blend.
Optional Flavor Boosters
- 1/4 teaspoon crushed red pepper flakes for gentle heat
- 1 teaspoon Dijon mustard in the sauce for tang
- 1/2 teaspoon Italian seasoning or dried thyme for an herby note
I often add Dijon and a pinch of red pepper flakes, because they cut through the richness and keep each bite interesting. You can also sprinkle fresh basil over the finished casserole if you have some in the garden.
Equipment List
- 9×13 inch baking dish or similar casserole dish
- Large skillet or sauté pan
- Medium saucepan for the cheese sauce
- Cutting board and sharp knife
- Whisk
- Wooden spoon or spatula
- Mixing bowl for topping
- Aluminum foil
Tips & Tricks
- Salt the sliced squash and zucchini lightly, then let them sit 10 to 15 minutes and pat dry to reduce excess moisture.
- Slice vegetables evenly so they cook at the same rate and keep a pleasant texture.
- Sauté the onion and garlic until they turn lightly golden to build deeper flavor.
- Shred cheese from a block when possible, since it melts smoother and tastes sharper.
- Warm the milk slightly before adding it to the roux so the sauce thickens faster and stays silky.
- Taste the cheese sauce and adjust salt, pepper, and spices before you pour it over the vegetables.
- Cover the casserole for the first part of baking so the veggies soften, then uncover to brown the top.
- If the top browns too quickly, tent the dish loosely with foil and move it to a lower oven rack.
- Let the casserole rest 10 minutes after baking so it sets and slices neatly.
- Add cooked shredded chicken or rotisserie chicken if you want extra protein in the dish.
How to Make Cheesy Summer Squash and Zucchini Casserole
Step 1: Prep the Vegetables and Oven
Preheat your oven to 375°F and grease a 9×13 inch baking dish with butter or oil. Wash the summer squash and zucchini, trim the ends, and slice them into 1/4 inch rounds. Place the slices in a colander, sprinkle with a little salt, and let them sit while you prep the rest.
After 10 to 15 minutes, pat the squash and zucchini dry with paper towels. This step pulls out some moisture so the casserole bakes creamy instead of watery. Slice the onion thinly and mince the garlic so they cook quickly and evenly.
Step 2: Sauté the Veggies
Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the sliced onion and cook until it turns soft and lightly golden around the edges, about 5 to 7 minutes. Stir in the minced garlic and cook 30 seconds until it smells fragrant.
Add the sliced squash and zucchini to the skillet and toss to coat them in the oil and onion mixture. Cook 5 to 7 minutes, stirring occasionally, until the vegetables start to soften but still feel slightly firm in the center. Season with a pinch of salt and pepper, then transfer the mixture to the greased baking dish and spread it into an even layer.
Step 3: Make the Cheesy Sauce
In a medium saucepan, melt 3 tablespoons of butter over medium heat. Whisk in the flour and cook 1 to 2 minutes, whisking constantly, until the mixture turns slightly golden and smells nutty. This step cooks out the raw flour taste and sets you up for a smooth sauce.
Slowly pour in the milk and cream while you whisk, and keep whisking until the mixture looks smooth. Add the salt, pepper, garlic powder, onion powder, smoked paprika, and Dijon if you use it. Bring the sauce to a gentle simmer and cook 3 to 5 minutes, whisking often, until it thickens enough to coat the back of a spoon.
Turn the heat to low and stir in 1 cup of the shredded cheddar, all the mozzarella, and 1/4 cup Parmesan. Stir until the cheese melts and the sauce looks smooth and glossy. Taste and adjust seasoning, adding more salt, pepper, or red pepper flakes if you want extra kick.
Step 4: Assemble the Casserole
Pour the hot cheese sauce evenly over the sautéed squash, zucchini, and onions in the baking dish. Use a spatula to nudge the sauce into the corners and between the vegetables so everything gets coated. Sprinkle the remaining 1/2 cup cheddar over the top.
In a small bowl, mix the panko breadcrumbs, 2 tablespoons Parmesan, and melted butter or olive oil. Stir until the crumbs look evenly coated and slightly clumpy. Scatter this mixture over the top of the casserole in an even layer.
Step 5: Bake Until Bubbly and Golden
Cover the baking dish loosely with foil and place it on the middle rack of the oven. Bake 20 minutes so the vegetables finish softening and the sauce bubbles. Remove the foil and bake another 10 to 15 minutes, until the top turns golden and crisp and you see bubbling around the edges.
If you want extra color on the topping, switch the oven to broil for 1 to 2 minutes and watch it closely. Pull the casserole from the oven and let it rest 10 minutes before serving. This rest time helps the cheesy summer squash and zucchini casserole set up so you scoop neat portions instead of cheesy lava.
What to Serve with it?
This cheesy summer squash and zucchini casserole pairs nicely with simple grilled chicken, baked fish, or roasted turkey breast. You can also serve it with a crisp green salad, sliced tomatoes with a drizzle of olive oil and salt, or steamed green beans. If you want a heartier vegetarian plate, add some quinoa, brown rice, or crusty bread on the side. I also like to pour tall glasses of iced tea, lemonade, or sparkling water with lemon to keep the meal light and refreshing.
Storage Options
- Store leftovers in an airtight container in the fridge for up to 4 days.
- Reheat single portions in the microwave in 45 to 60 second bursts until hot, then crisp the top under the broiler for a minute if you want crunch.
- Reheat larger portions in a 350°F oven, covered, for 15 to 20 minutes, then uncover for 5 minutes to refresh the topping.
- Freeze cooled casserole in a freezer-safe container or tightly wrapped dish for up to 2 months, then thaw overnight in the fridge before reheating in the oven.