How to Make It
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.