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A close-up of a cheesy, golden-brown baked squash casserole in a white oval dish with a serving spoon taking a scoop.

Creamy Zucchini and Yellow Squash Casserole

A delightful and comforting casserole featuring tender slices of zucchini and yellow squash baked in a rich, cheesy sauce and topped with a golden, crispy breadcrumb crust. Perfect as a satisfying side dish for any meal.
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Prep Time 20 minutes
Cook Time 35 minutes
Total Time 55 minutes
Servings: 6 servings
Course: Side Dish
Cuisine: American
Calories: 350
Ingredients Equipment Method Notes

Ingredients
  

For the Squash
  • 2 lbs zucchini and/or yellow squash sliced into 1/4-inch rounds
  • 1 tbsp olive oil
  • 1/2 tsp salt
  • 1/4 tsp black pepper
For the Cheese Sauce
  • 2 tbsp unsalted butter
  • 1/4 cup all-purpose flour
  • 2 cups milk whole or 2%
  • 1/2 cup sour cream
  • 1 tsp garlic powder
  • 1/2 tsp onion powder
  • 1/4 tsp smoked paprika optional, for depth of flavor
  • Salt and black pepper to taste
For the Topping
  • 1.5 cups shredded sharp cheddar cheese divided
  • 1/2 cup grated Parmesan cheese divided
  • 1 cup panko breadcrumbs
  • 2 tbsp unsalted butter melted

Equipment

  • Large Mixing Bowl
  • 9x13 inch baking dish
  • Large skillet or saucepan
  • Whisk
  • Measuring cups and spoons

Method
 

Prepare the Squash
  1. Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
  2. In a large mixing bowl, toss the sliced zucchini and/or yellow squash with 1 tablespoon of olive oil, 1/2 teaspoon of salt, and 1/4 teaspoon of black pepper until all slices are evenly coated.
  3. Arrange the seasoned squash in a single layer (or slightly overlapping) in the prepared baking dish. Bake for 15 minutes to partially cook the squash and allow it to release some moisture. Carefully drain any excess liquid from the dish and set the squash aside.
Make the Cheese Sauce
  1. While the squash is baking, melt 2 tablespoons of unsalted butter in a large skillet or saucepan over medium heat.
  2. Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a smooth roux. Do not let it brown.
  3. Gradually whisk in the milk until the sauce is smooth and lump-free. Bring the sauce to a gentle simmer, stirring constantly, until it thickens (this usually takes about 3-5 minutes).
  4. Remove the skillet from the heat. Stir in the sour cream, garlic powder, onion powder, and smoked paprika (if using). Season with additional salt and black pepper to taste.
  5. Add 1 cup of the shredded sharp cheddar cheese and 1/4 cup of the grated Parmesan cheese to the sauce. Stir until the cheeses are completely melted and the sauce is smooth and creamy.
Assemble and Bake
  1. Pour the prepared cheese sauce evenly over the partially baked squash in the baking dish. Gently toss the squash with the sauce to ensure all pieces are coated, or simply spread the sauce over the top.
  2. In a small bowl, combine the panko breadcrumbs, 2 tablespoons of melted butter, the remaining 1/2 cup of shredded sharp cheddar cheese, and the remaining 1/4 cup of grated Parmesan cheese. Mix well until the breadcrumbs are evenly coated.
  3. Evenly sprinkle the cheesy breadcrumb mixture over the top of the casserole.
  4. Bake for 20-25 minutes, or until the casserole is bubbly around the edges and the breadcrumb topping is golden brown and crisp.
Serve
  1. Let the casserole rest for 5-10 minutes before serving. This allows the sauce to set slightly and makes for easier serving.

Notes

For an extra layer of flavor, consider sautéing a small diced onion in the butter before adding the flour for the sauce. This casserole can be assembled up to a day ahead of time (excluding the breadcrumb topping), covered, and refrigerated. Add the topping just before baking. Leftovers store well in an airtight container in the refrigerator for 3-4 days and can be reheated in the microwave or oven.