Place the teriyaki sauce, garlic, sesame oil, sugar and pepper in a saucepan and simmer gently until the sugar dissolves. Let cool.
Preheat a grill plate until smoking hot. Seal the beef fillets quickly on both sides and place in an airtight container and cover with the teriyaki sauce. Marinate in the fridge up to three days, turning the beef occasionally.
To serve, preheat the oven to 200 degrees C. Place beef (drained of the sauce) on a baking tray and cook 8 minutes. Meanwhile, simmer the marinade until reduced by half. Serve the fillets with the marinade as sauce.