Combine the Soya sauce, sake or mirin, rice vinegar, sesame oil, garlic and pepper in a bowl and mix well.
Place the steak on a plate and pour the marinade over it; let marinate for 4 to 6 hours in the refrigerator.
Drain the steak well and set aside the marinade.
Grill for 6 to 9 minutes on each side (avoid overcooking or the steak will become tough).
Heat the marinade in a saucepan over low heat and thicken with the cornstarch.
When serving, slice the steak against the grain and accompany with the sauce.