For a tasty and colorful twist, substitute sliced zucchini for half of the yellow squash.
Preheat oven to 350°. Cook yellow squash and onion in boiling water to cover in a Dutch oven 8 minutes or just until tender; drain squash mixture well.
Combine squash mixture, Cheddar cheese, next 5 ingredients, 1 cup breadcrumbs, and 3/4 cup Parmesan cheese. Spoon into a lightly greased 13- x 9-inch baking dish.
Stir together melted butter and remaining 1 1/2 cups breadcrumbs and 1/2 cup Parmesan cheese. Sprinkle breadcrumb mixture over top of casserole.
Bake at 350° for 35 to 40 minutes or until set.
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