In a saucepan, combine the soy sauce, mirin, sake, sugar, scallions, ginger, garlic and 2 teaspoons of the sesame oil. Bring to a boil over moderate heat and cook until reduced to 1 1/4 cups, 8 to 10 minutes. Strain the sauce into a small bowl.
Light a grill or preheat the broiler. Brush the steak with the remaining 1 tablespoon of sesame oil and season generously with salt and pepper. Grill or broil the steak, basting with some of the sauce, for 4 to 5 minutes per side for medium rare. Let rest for 5 minutes, then thinly slice the steak on the diagonal and serve with the Sweet Potato Packets. Pass the remaining sauce separately.
WINE
The acerbic qualities of such classic Bordeaux as the 1995 Château Coufran and the 1994 Château Grand-Puy Ducasse mirror the salty soy sauce.