1. 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.
2. 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.
3. Stir together melted butter and remaining 1 1/2 cups breadcrumbs and 1/2 cup Parmesan cheese. Sprinkle breadcrumb mixture over top of casserole.
4. Bake at 350° for 35 to 40 minutes or until set.