Preheat oven to 350°. Grease casserole dish.Prepare macaroni and cheese according to package. Brown ground beef with celery, green pepper and onions, salt and pepper Drain. Add hamburger mixture to macaroni and cheese and stir until blended.Top with 1/2 shredded cheddar cheese.Bake for 15 to 20 minutes or until lightly browned on top.