Combine ground beef with chopped onion, egg, bread crumbs, salt and pepper. Shape mixture into about 24 meatballs, then brown in margarine. Drain well. Stire together tomatoes, water, beef granules and seasonings. Place carrots, potatoes, and sliced onion in bottom of crockpot, top with meatballs. Pour tomatoe mixture over all. Cover and cook on low setting for 8 to 10 hours. Before serving, remove meatballs with a slotted spoon. Make a smooth paste of the cornstarch and water and stir into vegetables. Cover and cook on high setting for 10 minutes to thicken. Add meatballs to stew and serve.