Combine soy sauce with sake, granulated garlic, and black pepper for the steak. Add the steak and marinate for 20 minutes at room temperature.
Meanwhile, prepare the sauce. Toss scallions in 1 tablespoon oil and lay in one layer on the grill. Cook until lightly charred on both sides, about 3-4 minutes. Remove from the grill and roughly chop scallions.
Combine scallions with walnuts, 6 tablespoons olive oil, garlic, sherry vinegar, and sugar in a food processor or blender. Puree until the mixture becomes a thick paste. Season with salt and pepper and set aside.
Lay steaks on the grill and cook until the desired degree of doneness.
Remove steaks from the oven and let rest for 10 minutes. Slice and serve with scallion sauce.
Notes
The scallion walnut sauce can be prepared the day before.