How to make it
For the Karashi Soy Sauce: Add all liquids to saucepan and bring to a boil. Boil for about two minutes, remove from heat and let the sauce cool down to room temperature. Once the sauce has cooled down, mix in the mustard.
Preheat the grill to medium-hot heat. Marinate 4 1-inch thick sirloin steaks in the Garlic-Soy Marinade for about five minutes.
Grill the steaks on both sides to medium-rare (about four to five minutes on the first side and three to five minutes on the second), remove from heat and let rest for about three minutes.
Slice the steak into thin pieces and serve with the Karashi Soy Sauce. Garnish with sesame seeds and sliced scallion, if desired.