recipe squash casserole

Ingredients 2 pounds yellow squash, sliced 1 small onion, chopped 1 cup grated cheddar cheese 1/2 cup grated Parmesan cheese 1 cup sour cream 1 can (10.5 ounces) cream of mushroom soup 2 large eggs, beaten 1 teaspoon salt 1/2 teaspoon black pepper 1/2 teaspoon garlic powder (optional)