Combine onion, green pepper, ground beef and pepperoni in a lg. skillet. Cook over med. heat until beef browns, stirring to crumble. Remove from the heat, drain. Return to skillet and add sauce and bring to a boil. Cover, reduce heat and simmer 20 min. stirring occasionally.
Cook spaghetti as directed, omit salt. Drain. Arrange in a casserole and spoon meat sauce on top of noodles. Sprinkle with cheeses and bake at 400 for 3-5 min. Remove from the oven and serve immediately.