This cheesy baked zucchini casserole is absolutely delicious and perfect as a comforting vegetable side.
I’m forever experimenting with zucchini — it’s versatile, mild, and so easy to transform. I love it raw, roasted, spiralized, or baked, and this casserole is one of my favorite ways to spotlight its tender texture and subtle flavor.
This recipe takes zucchini into cozy casserole territory using a rich caramelized onion cream sauce and a mix of melty cheeses. The result is a warm, cheesy side with a crisp, buttery breadcrumb topping — perfect for family dinners, potlucks, or any time you want comfort food without meat.
The recipe below makes about eight side-dish servings. If you prefer fewer leftovers, halve the ingredients to make a smaller casserole for two to four people.
Baked Zucchini Casserole
This easy, cheesy vegetarian side is hearty, flavorful, and simple to prepare.
Zucchini Casserole
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Ingredients
- 1 lb zucchini squash
- 2 cups finely diced onion
- 1 tsp olive oil
- 3 TBSP butter
- 3 TBSP all-purpose flour
- ¾ cup milk (warmed)
- ¼ tsp salt
- ¼ tsp black pepper
- ¼ tsp garlic powder
- ⅛ tsp ground nutmeg
- 4 oz freshly grated Gruyere cheese
- 4 oz freshly grated Gouda cheese
Crispy Italian Breadcrumb Topping
- ½ cup Italian seasoned breadcrumbs (panko or regular)
- 1 TBSP unsalted butter
Instructions
Prep Work
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Measure all ingredients before you begin — the sauce comes together quickly.
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Trim the ends and slice zucchini into 1/4-inch rounds. Avoid slices that are too thin to prevent them becoming mushy. For smaller bites, halve each round lengthwise.
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Sprinkle the zucchini slices with sea salt and place them on an absorbent clean dish towel. Let sit at least 15 minutes; 30–40 minutes pulls more moisture out. When beads of water appear on the surface, pat slices dry on both sides with paper towels to remove as much moisture as possible.
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Preheat the oven to 375°F (190°C).
Get Cooking!
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Heat a medium pot over medium-high heat with 1 tsp olive oil. Sauté the diced onions, stirring occasionally, until they are golden brown and tender.
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Push the onions to the side, add 3 TBSP butter and let it melt. Stir in the flour and whisk constantly for about 2 minutes until the roux darkens slightly and bubbles. Gradually pour in the warmed milk while whisking, bring to a light boil, and whisk for about 1 minute until the sauce thickens.
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Remove the pot from heat. Season with salt, pepper, garlic powder, and nutmeg. In 2–3 batches, whisk in half the Gruyere and all the Gouda until the cheeses melt smoothly into the sauce.
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Arrange the zucchini in a medium baking dish, pour the cheese sauce over the slices, and sprinkle the remaining Gruyere on top. Bake at 375°F for about 20–25 minutes, until the top is golden and the zucchini is tender.
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While the casserole bakes, toast the Italian-seasoned breadcrumbs in a skillet with 1 TBSP unsalted butter until golden and crisp. Scatter the toasted crumbs over the casserole and return to the oven for the remaining bake time if needed.
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Serve hot and enjoy.
Notes
Nutrition facts are estimates from a recipe nutrition calculator and may vary based on brands and portion sizes.
Nutrition
If you try this cheesy baked zucchini casserole, I’d love to hear how it turned out. Leave a comment or tag your photos so you can share your version. It’s always exciting to see how others customize it.
Have a few extra zucchini to use up?
There are plenty of other tasty zucchini ideas: crispy zucchini parmesan, hasselback zucchini pizzas, or zucchini-studded veggie burgers are great ways to use a surplus and keep meals interesting. These options are crowd-pleasing and make use of garden-fresh squash in different, satisfying ways.