Cut tops off peppers and remove seeds. Place peppers in a large kettle and cover with water. Bring to a boil; cook until crisp-tender, about 3 minutes. Drain and rinse in cold water; set aside.
In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Add the beans, tomatoes, chili powder, salt if desired, pepper and cayenne. Bring to a boil. Reduce heat; cover and simmer for 5 minutes.
Spoon meat mixture into peppers; place in an ungreased 3-qt. baking dish. Cover and bake at 350° for 20-25 minutes or until heated through. Sprinkle with cheese. Yield: 6 servings.