In 3 1/2 to 4-quart slow cooker, combine meatballs, onions, salt and tomatoes; mix well.
Cover; cook on low setting for 6 to 8 hours.
About 20 minutes before serving, in small bowl, blend flour and water until smooth. Stir flour mixture into stew. Stir in bell pepper and onion stir-fry. Increase heat setting to high; cover and cook an additional 15 to 20 minutes or until stew has thickened and bell peppers are thoroughly heated. Sprinkle individual servings with cheese.