Cheesy Summer Squash and Zucchini Casserole

Cheesy Summer Squash and Zucchini Casserole tastes like creamy, garlicky mac and cheese met a bubbly veggie gratin and decided to show off. It works perfectly for busy weeknights, potlucks, or holiday tables, and you can get it on the table in about 50 minutes. I first tested this version during a heatwave with a cranky toddler and a tiny oven, so I promise it earns its spot in your rotation.

Why You Should Try This Cheesy Summer Squash and Zucchini Casserole

This Cheesy Summer Squash and Zucchini Casserole turns simple garden veggies into serious comfort food. The squash stays tender, the cheese melts into a rich sauce, and the top bakes into a golden, slightly crisp layer.

The recipe works for picky eaters, veggie lovers, and anyone who stares at a pile of zucchini and feels slightly overwhelmed. You can serve it as a vegetarian main or as a hearty side, and it fits right in at cookouts, Sunday dinners, or meal prep.

“This Cheesy Summer Squash and Zucchini Casserole tastes like summer comfort in a dish, and my whole family scraped the pan clean. ★★★★★”

Ingredients You’ll Need

Fresh produce

• 2 medium yellow summer squash, thinly sliced into rounds
• 2 medium zucchini, thinly sliced into rounds
• 1 small yellow onion, thinly sliced
• 3 cloves garlic, minced (use jarred minced garlic if you like shortcuts)
• 2 tablespoons fresh parsley, chopped (or 1 teaspoon dried)
• Optional: 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme

Dairy and eggs

• 1 1/2 cups shredded sharp cheddar cheese
  • I like Tillamook or Cabot for strong flavor.
• 1 cup shredded mozzarella cheese
  • Pre-shredded works fine here and saves time.
• 1/2 cup grated Parmesan cheese
  • Use the real stuff from a wedge if possible, not the green can.
• 1 cup whole milk or half-and-half
• 2 large eggs

Pantry staples

• 3 tablespoons olive oil or melted butter
• 1/2 cup panko breadcrumbs
  • Regular breadcrumbs work, but panko gives a better crunch.
• 2 tablespoons all-purpose flour
  • Use a 1:1 gluten free flour blend if you need a gluten free option.
• 1 teaspoon kosher salt, plus more to taste
• 1/2 teaspoon black pepper
• 1/2 teaspoon smoked paprika or sweet paprika
• 1/4 teaspoon red pepper flakes (optional, for a little heat)

Optional add-ins

• 1/2 cup cooked crumbled bacon
• 1/2 cup cooked quinoa or rice for extra heft
• 1/2 cup sautéed mushrooms if you want more veggies

Equipment list

• 9×13 inch baking dish or similar casserole dish
• Large skillet
• Large mixing bowl
• Small bowl for breadcrumb topping
• Whisk
• Sharp knife and cutting board
• Aluminum foil
• Oven preheated to 375°F

Tips & Tricks

• Slice the squash and zucchini evenly so they cook at the same rate and stay tender, not mushy.
• Salt the sliced squash and zucchini lightly, let them sit 10 to 15 minutes, then pat dry to reduce extra moisture.
• Use a mix of sharp cheddar and mozzarella so you get both flavor and stretch.
• Shred cheese from a block when you can, since it melts smoother than bagged cheese.
• Layer the veggies slightly overlapping, like shingles, so the casserole holds together and looks pretty.
• Cover the casserole with foil for the first part of baking so the veggies soften, then uncover to brown the top.
• If the top browns too fast, tent loosely with foil near the end.
• Let the casserole rest 10 minutes before serving so it sets and slices cleanly.
• Taste the cheese mixture before you add eggs and adjust salt and pepper so the flavor pops.
• Use a baking sheet under the dish if you fill it to the top, so any bubbling stays off the bottom of your oven.

How to Make Cheesy Summer Squash and Zucchini Casserole

Prep the veggies

1. Preheat your oven to 375°F and grease your baking dish with a little olive oil or butter.
2. Slice the summer squash and zucchini into thin rounds, about 1/8 to 1/4 inch thick.
3. Place the slices in a colander, sprinkle with a pinch of salt, and let them sit 10 to 15 minutes.
4. Pat the slices dry with paper towels to remove extra moisture so the casserole does not turn watery.

Sauté the aromatics

1. Heat 1 tablespoon olive oil in a large skillet over medium heat.
2. Add the sliced onion and cook 5 to 7 minutes until it turns soft and lightly golden.
3. Stir in the minced garlic and cook 30 seconds until fragrant, then turn off the heat.
4. Set the skillet aside so the mixture cools slightly.

Mix the cheesy custard

1. In a large mixing bowl, whisk the eggs until smooth.
2. Add the milk or half-and-half, flour, salt, pepper, smoked paprika, and red pepper flakes if you use them.
3. Whisk until the flour blends in and no lumps remain.
4. Stir in the cheddar, mozzarella, half of the Parmesan, parsley, thyme, and the sautéed onion and garlic.
5. Taste a tiny bit of the mixture and adjust seasoning with more salt or pepper if needed.

Assemble the casserole

1. Spread a thin layer of the cheese mixture on the bottom of the greased baking dish.
2. Arrange a layer of squash and zucchini slices over it, slightly overlapping.
3. Spoon some of the cheese mixture over the veggies, spreading it gently with the back of the spoon.
4. Repeat the layers until you use all the veggies and cheese mixture, finishing with cheese on top.
5. If you use bacon, mushrooms, or cooked grains, tuck them between layers of veggies and cheese.

Add the crunchy topping

1. In a small bowl, mix the panko breadcrumbs, remaining Parmesan, and 2 tablespoons olive oil or melted butter.
2. Stir until the crumbs look evenly coated and slightly clumpy.
3. Sprinkle the breadcrumb mixture evenly over the top of the casserole.

Bake to bubbly perfection

1. Cover the baking dish loosely with foil, tenting it so it does not touch the topping.
2. Bake at 375°F for 20 minutes.
3. Remove the foil and bake another 15 to 20 minutes until the top turns golden and the edges bubble.
4. Check the center with a knife; the squash should feel tender when you poke it.
5. Let the casserole rest 10 minutes on the counter so it sets before you scoop or slice.

What to Serve with Cheesy Summer Squash and Zucchini Casserole

This Cheesy Summer Squash and Zucchini Casserole pairs nicely with simple grilled chicken, roasted salmon, or a big green salad. I like to add sliced tomatoes with a little salt and olive oil on the side to keep the plate fresh and bright. You can also serve it with crusty bread or garlic toast to catch every bit of cheesy sauce. If you cook for kids, add some fruit salad or apple slices and call it a complete, cozy meal.

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 let them sit 1 minute.
• Reheat larger portions in a 350°F oven, covered, for 15 to 20 minutes, then uncover a few minutes to crisp the top.
• Freeze cooled portions in freezer safe containers for up to 2 months, and thaw overnight in the fridge before reheating.