Preheat oven to 350 degrees F.
Combine soy sauce with garlic, sake and black pepper for the steaks. Add the steaks and marinate for 15 minutes at room temperature.
Meanwhile, prepare the sauce. Heat 1 tablespoon oil in a saute pan over medium-high heat. Add the scallions and saute until charred on both sides, about 5 minutes. Remove from heat. Chop scallions and combine with walnuts, garlic, sherry vinegar, mustard and pinch of sugar. Slowly add the olive oil, whisking constantly. Season with salt and pepper and set aside.
Heat 1 tablespoon oil in a saute pan over medium-high heat. Add the steaks and cook until browned, about 5 minutes. Flip and cook on the other side. Finish cooking in the oven until the desired degree of doneness.
Remove steaks from the oven and let rest 10 minutes. Slice and serve with scallion sauce.