Squash and Zucchini Casserole

This squash and zucchini casserole is a cozy side dish starring tender squash in a light Parmesan sauce with a golden, crunchy breadcrumb topping.

When I’ve got loads of squash and zucchini on hand in peak summer, here’s one of my favorite things to do with them: make this squash and zucchini casserole! It’s a cozy casserole that’s not too rich and heavy, and every time I make it, I can’t stop shoveling in bites.

This one is packed with flavor, with a light Parmesan sauce that covers the tender squash topped with crunchy breadcrumbs. It’s such an easy and unique summer side dish!

Ingredients You’ll Need

This squash and zucchini casserole has a short ingredient list! There’s only one special ingredients with some notes. Here’s what you’ll need:

Summer squash and zucchini: Weigh out the amount so that you use the right amount for the seasoning. Medium zucchini and squash can vary in weight greatly.
Spices: Garlic powder, onion powder, dried oregano, and kosher salt combine for big flavor (no chopping an onion required!).
Olive oil or butter: Either works; butter adds richness, olive oil keeps it lighter.
Parmesan cheese: This aged cheese flavors both the sauce and the topping.
2% milk and flour: These make the light, creamy sauce.
Seasoned Italian panko: This is the bread crumb mixture that contains seasonings and salt! You should be available to find it at your local grocery. If not, see the notes below.

Want more of this duo? Try Roasted Zucchini and Squash or Grilled Zucchini and Squash.

The Italian Panko Trick (and a Substitute)

This casserole is made using Italian panko or breadcrumbs, which have seasonings and salt added. This makes it easy to sprinkle onto the top of a casserole because it’s got lots of flavor and seasoning already. Italian breadcrumbs and panko are easy to find at most grocery stores. If you can’t find it, here’s a substitute:

Use regular panko or make homemade breadcrumbs.
To 1 cup of the plain panko or breadcrumbs, add: ¼ teaspoon dried basil, ¼ teaspoon dried oregano, ⅛ teaspoon garlic powder and ⅛ teaspoon dried thyme, and ⅛ teaspoon kosher salt.
Use ½ cup in the recipe below and save the remaining for future recipes.

How to make bread crumbs with herbs — Try homemade Italian breadcrumbs if you can’t find storebought

How to Make Squash and Zucchini Casserole

The basic steps for making this squash and zucchini casserole! There are a few technique things to know, so review this first before going to the recipe below:

Slice and season the squash and zucchini. Three pounds might seem like a lot of squash and zucchini at first, but it condenses into a small baking dish. Slice it into rounds and season it in a bowl first to get even coating.
Cook the squash and zucchini: You’ll need your largest skillet for this one! The easiest way to cook it all is by covering it for a few minutes at a time. A lot of moisture will be released. Cook only until crisp tender: it takes about 6 minutes.
Add the sauce: To the same pan, you’ll add milk, flour whisked into a little milk (as a thickener), and Parmesan cheese. Let that cook for 2 minutes: it will thicken a little, but not much.
Top with breadcrumbs and bake! Finally, you’ll transfer everything to a small baking dish, top with breadcrumbs, and bake for 10 minutes.

How to Keep Squash Casserole From Getting Watery

Watery casserole is the biggest complaint with this style of side dish, since zucchini and squash contain so much water. Here are the keys to keeping it set:

Pre-cook the zucchini and squash. Sautéing first removes much of the moisture from these veggies. Make sure to cook only to crisp tender.
Bake hot and fast. This allows the veggies to be tender without overcooking them.
Rest for 10 minutes after baking to let the sauce in the pan thicken. When the casserole is just baked, the sauce is fairly watery. As it cools, the sauce will thicken. Also, the squash and zucchini are very hot once they come out of the oven. The rest time helps them come to warm and helps to set the texture.

Variation: gluten free casserole

Do you eat gluten-free? Here are a few options for making this a gluten free squash and zucchini casserole:

Panko / breadcrumbs: Gluten free Italian breadcrumbs and panko are easily available online these days. You can also sometimes find them at your local grocery. (If all you can find is plain, not Italian, see the section above about how to season your own.)
Flour: You can use gluten-free flour or cornstarch in place of the flour.

Ways to Serve Squash and Zucchini Casserole

This squash and zucchini casserole is easy to make! For us it’s more of a side dish for entertaining or a special meal, but you could also serve it on a weeknight if you’re an organized cook. Here are a few ways to serve it:

Fish: It would be great with Grilled Salmon, Grilled Salmon in Foil, Pesto Salmon, Grilled Tilapia, Baked Salmon, Baked Cod, or Easy Pan Fried Cod.
Shellfish: Try it with Cajun Shrimp, Grilled Shrimp or Seared Scallops with Lemon Herb Sauce.
Pasta: Try it with Tomato Basil Pasta, Pesto Spaghetti, or Pesto Cavatappi.

Storage and Make-Ahead Tips

You can refrigerate leftovers up to 3 days in a covered container. Reheat in a 350°F oven until warmed through. I would not recommend freezing leftovers, because they become watery when thawed.

To make this casserole ahead, prep this through the sauce step and refrigerate in the baking dish, then add the breadcrumb topping just before baking so it stays crunchy. Add a few extra minutes of bake time if needed since you’re baking from cold.

Dietary Notes

This squash and zucchini casserole is vegetarian. For gluten free, use gluten free Italian breadcrumbs or panko for the panko, and cornstarch instead of flour.