Zucchini Casserole Recipe: Cheesy Baked Zucchini with Crunchy Topping

By Aquarium Guides Editorial Team · Updated November 10, 2026

Last Updated: November 10, 2026

This zucchini casserole layers sautéed zucchini and onion in a creamy cheese sauce, then bakes it under a golden breadcrumb and Parmesan topping. It is a side dish that serves 6 to 8, works with nearly any roast or grilled protein, and can be assembled ahead of time. The key is cooking some of the water out of the zucchini first so the sauce stays thick instead of turning soupy.

The Problem with Watery Casseroles

Zucchini is about 95 percent water, and it releases most of it when heated. If you simply layer raw slices in a dish and bake, the liquid pools at the bottom and thins the sauce. Two steps fix this: salting the slices for 15 minutes to draw out moisture, and browning them in a skillet before assembling. Both take little effort and make a huge difference in texture.

Ingredients

Casserole

• 2 1/2 pounds zucchini (about 4 medium), sliced 1/4 inch thick
• 1 teaspoon kosher salt, for the zucchini
• 2 tablespoons olive oil
• 1 medium yellow onion, diced
• 3 cloves garlic, minced
• 3 tablespoons unsalted butter
• 3 tablespoons all-purpose flour
• 1 1/4 cups whole milk
• 1/2 teaspoon salt
• 1/4 teaspoon black pepper
• 1/4 teaspoon ground nutmeg
• 1 1/2 cups shredded sharp cheddar cheese, divided
• 1/2 cup shredded mozzarella cheese

Topping

• 1 cup panko breadcrumbs
• 1/3 cup grated Parmesan cheese
• 2 tablespoons melted butter
• 1/2 teaspoon dried thyme or Italian seasoning

Instructions

1. Heat the oven to 375°F. Lightly grease a 9×13-inch baking dish or a 2-quart casserole.
2. Place the zucchini slices in a colander, sprinkle with 1 teaspoon kosher salt, toss, and let drain for 15 minutes. Pat dry thoroughly with paper towels.
3. Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add half the zucchini in a single layer and cook 3 to 4 minutes per side until lightly browned. Transfer to a plate and repeat with the remaining oil and zucchini.
4. In the same skillet, reduce the heat to medium and cook the onion for 5 minutes until soft. Add the garlic and cook 30 seconds. Transfer the onions to the plate with the zucchini.
5. Melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook 1 minute. Slowly pour in the milk, whisking constantly, and cook 3 to 4 minutes until thickened.
6. Remove from the heat and stir in the salt, pepper, nutmeg, 1 cup of the cheddar, and the mozzarella until melted.
7. Fold the zucchini and onions into the sauce. Spread the mixture in the baking dish and top with the remaining 1/2 cup cheddar.
8. Combine the panko, Parmesan, melted butter, and thyme. Sprinkle evenly over the casserole.
9. Bake uncovered for 25 to 30 minutes, until bubbling at the edges and golden on top. Broil for 1 to 2 minutes if you want deeper browning.
10. Rest for 10 minutes before serving so the sauce sets.

Tips

• Salt and dry. Do not skip the draining step. Pat the slices hard with towels, since any surface moisture goes into the sauce.
• Brown in batches. Crowding the skillet steams the zucchini instead of browning it.
• Shred cheese yourself. Pre-shredded cheese contains anti-caking starch and melts less smoothly.
• Keep the sauce thick. It should coat a spoon before you add the zucchini, because the vegetables will thin it slightly while baking.
• Rest before serving. A 10-minute rest makes the casserole sliceable rather than runny.

Variations

Zucchini and Tomato Casserole

Layer in thin slices of tomato between zucchini, patted dry, and replace the cheddar with fresh mozzarella and a little basil.

Sausage Zucchini Casserole

Brown 1 pound Italian sausage, drain, and fold it in with the zucchini for a main dish.

Cracker Topping

Swap the panko for crushed buttery crackers for a richer crunch.

Squash Mix

Replace half the zucchini with yellow summer squash. They cook at the same rate.

Spicy Version

Add 1/2 teaspoon red pepper flakes and a diced jalapeño sautéed with the onion, and use pepper jack cheese in place of cheddar.

Make-Ahead and Storage

Assemble the casserole without the topping, cover, and refrigerate for up to 24 hours. Add the topping just before baking, and extend the baking time by 5 to 10 minutes if the dish goes into the oven cold. Leftovers keep in the refrigerator for 3 to 4 days in an airtight container. Reheat individual portions in a 350°F oven for 15 minutes to re-crisp the topping. The microwave is faster but softens the crumbs.

Freezing is possible, but the zucchini texture becomes softer after thawing. If you freeze it, leave off the topping, wrap tightly, and use within 2 months. Thaw overnight in the refrigerator and add fresh topping before reheating.

What to Serve With It

This casserole pairs well with roast chicken, grilled pork chops, baked salmon, or a simple steak. It also works as a vegetarian main when served alongside a green salad and crusty bread.

Choosing and Prepping Zucchini

Pick small to medium zucchini that feel heavy for their size, with smooth, glossy skin. Large zucchini have bigger seeds and a spongier texture that releases more water. Trim the stem and blossom ends, then slice the zucchini into even rounds about a quarter inch thick. Uniform slices cook at the same rate, so a mandoline or a steady knife hand helps. If your zucchini are oversized, halve them lengthwise and scoop out the seedy center with a spoon before slicing into half-moons.

Cheese Guide

Scaling the Recipe

For a smaller side dish, halve everything and bake in an 8-inch square dish for 20 to 25 minutes. To feed a crowd, double the recipe and use two dishes rather than one deep pan, since a thick casserole takes longer to heat through and the topping may brown before the center is hot. Keep the sauce quantity proportional so the zucchini stays lightly coated.

FAQ

Do I need to peel the zucchini?

No. The skin softens during baking and helps the slices hold their shape.

Why is my casserole watery?

Usually the zucchini was not salted, dried, and browned first, or the sauce was too thin. Cook the sauce until it coats a spoon.

Can I use a different cheese?

Yes. Gruyère, Monterey Jack, fontina, or Colby all melt well. A blend of a sharp cheese and a mild melting cheese gives the best balance.

Can I make it without flour?

Replace the flour sauce with 1 cup sour cream mixed with 1 cup shredded cheese and a beaten egg. The texture is more custard-like, but it bakes in the same time.

6 Best Yoga Mats for Hot Yoga (November 2026)

8 Best Cedar Closet Planks (November 2026)

6 Best Fishing Boats (November 2026)