Zucchini Au Gratin

1 white onion, sliced
2 lbs. zucchini, sliced (I also use yellow squash if I have it on hand)
1/2 cup butter, melted and divided
2 c. shredded mozzarella cheese
1/2 cup dry bread crumbs
1/4 c. grated Parmesan cheese

In a lightly greased baking dish, layer onion and squash. Drizzle with 1/4 cup butter, sprinkle with mozzarella cheese. In a separate bowl, combine remaining butter, bread crumbs and Parmesan cheese. Sprinkle crumb mixture evenly over the top. Bake, uncovered, at 350 degrees for about 40 minutes or until squash is tender. Serves 8.