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This squash and zucchini casserole is a total crowd pleaser! A cozy summer side dish, it’s topped with crunchy breadcrumbs and Parmesan.

Squash and zucchini go hand in hand in the summers. And what’s more homey than a squash and zucchini casserole? It’s got the vibe of Sunday dinners and summer evenings where the sun stretches into the late evening. This one is packed with flavor and quickly disappeared in our house. A light Parmesan sauce covers the tender squash, and it’s topped with crunchy breadcrumbs. It’s a cozy casserole that’s not too rich and heavy. Honestly, we couldn’t stop ourselves from shoveling in bites. Here’s what to know!

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

Ingredients for squash and zucchini casserole

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, oregano, salt
Olive oil or butter
Parmesan cheese
2% milk
Flour
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.

Love yellow squash? Check out these Yellow Squash Recipes to Try!

Use Italian Panko or breadcrumbs, or this substitute

This casserole is made using Italian panko or breadcrumbs. What are they? Italian breadcrumbs 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 (basic steps)

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 & 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.

Here’s why to rest after baking!

Once you’re done baking this squash and zucchini casserole, here’s the thing. It’s best to rest for at least 10 minutes after baking. Why? Two reasons:

Resting lets the sauce in the pan thicken. When the casserole is just baked, the sauce is fairly watery. As it cools, the sauce will thicken.
It also cools and sets the squash. 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. 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.

This squash and zucchini casserole is…

Vegetarian. For gluten free, see the section above.