Combine mirin (80 ml), sake (80 ml), soy sauce (80 ml), sugar (1 tbsp) and grated ginger (1 tbsp) in a shallow bowl. Marinate the steak at least 1 hour (up to 48 hrs, chilled).
Remove steak, reserve marinade. Boil marinade in a saucepan for 10 min until reduced to a thin glaze — this is your teriyaki sauce.
Heat grill (oiled grates) or cast iron pan on high. Pat the steak dry, rub with olive oil. Sear 3-5 min per side until well browned, basting with teriyaki sauce.
Remove, cover with foil, rest 10-15 min. Serve with rice.