Preheat oven to 350F. Peel and cut squash and slice. Bring about 1-1/2 quarts water to a boil; add squash. Cook 15 minutes, then drain. In 2-quart sauce pan, heat oil. Add bell pepper, onions, celery, and saute until vegetables are soft. Lower heat and add ground meat to mixture. Cook until meat is light brown, Add margarine and squash. Cook 15 minutes longer. Add 3/4 cup bread crumbs. Pour in 2 quart round casserole. Top with bread crumbs or Ritz crackers. Bake 15 to 20 minutes at 350F or until top is bubbly and light brown.