In a medium bowl, stir together the soy sauce, water, pepper and cornstarch. Add the meat, and marinate for at least 1/2 hour or overnight.
Preheat your oven's broiler. Heat the oil in a large skillet over medium heat. Add onion; cook and stir until browned. Remove from the skillet and set aside. Add beef strips to the skillet and quickly fry just until browned.
Place meat, cheese and onions onto buns, and quickly melt under the broiler.
