To start, prepare 2 pounds of zucchini and summer squash by washing, drying, and slicing them into ½ inch rounds. For better texture, you can sprinkle the slices with salt, let them sit for 10 minutes, and then pat them dry to remove excess moisture. This step is optional but can enhance the casserole's flavor.
In a large skillet, melt 2 tablespoons of butter over medium heat. Add 1 large chopped onion and sauté until softened and translucent, about 5 minutes. Then, add 3 cloves of minced garlic and cook for another minute. This creates a flavorful aromatic base for the casserole.
Next, add the sliced zucchini and summer squash to the skillet with the onions and garlic. Cook the vegetables for 5-7 minutes until they are tender-crisp, seasoning with a pinch of salt and pepper as they cook. This step ensures the vegetables are well-seasoned and cooked to the right texture.
In the same skillet, melt another 2 tablespoons of butter and whisk in 2 tablespoons of flour to create a roux. Cook the roux for 1 minute, then slowly whisk in 1 ½ cups of milk or broth. Bring the mixture to a simmer and cook until the sauce thickens slightly. Remove the skillet from heat and stir in ½ cup of grated Parmesan cheese, along with a pinch of nutmeg, salt, and pepper to taste. This creates a rich, creamy sauce that will coat the vegetables.
Preheat your oven to 375°F (190°C). Transfer the cooked vegetables to a greased 9x13 inch baking dish and pour the creamy sauce evenly over them. For the topping, you have two options: combine ⅓ cup panko breadcrumbs, ⅓ cup grated Parmesan cheese, and 2 tablespoons melted butter for a crispy topping, or mix 1 cup crushed Ritz crackers with ⅓ cup grated cheddar cheese for a cheesy crumble. Sprinkle your chosen topping over the casserole.
Bake the casserole in the preheated oven for 25-30 minutes, or until the topping is golden brown and the casserole is bubbly around the edges. This baking time ensures the flavors meld together beautifully and the topping provides a delightful textural contrast. Allow the casserole to cool slightly before serving, and enjoy this creamy, flavorful dish.
