Saute onions, green pepper, jalapeno pepper and garlic in oil until onions are translucent. Add beef and cook, stirring until meat is lightly browned. Drain off all fat, add salt, pepper, and cumin and cook for 1 min. more, stirring all the time to season evenly. Add ketchup and tomatoes and cook, uncovered, over low heat until reduced to a thick sauce. Serve on toasted buns.