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