In skillet, brown 1 pound ground beef; pour off fat. Meanwhile, cut 8 large zucchini in half; scoop out centers, leaving a thick wall. Chop centers and add to beef along with 1/2 cup fine dry bread crumbs and 1/4 cup chopped onion. Place zucchini "boats" in shallow baking dish; heap meat mixture in and around zucchini. Pour 2 cans (8 ounce each) or 1 can (15 ounce) tomato sauce with tomato bits over zucchini. Cover with aluminum foil; bake at 400 degrees for 45 minutes or until tender.