Make the teriyaki sauce:
Bring the soy sauce, brown sugar, ginger, and garlic to a boil over medium-high heat in a small saucepan. Boil until the mixture has thickened slightly, about 2 minutes. Stir in 1/2 tsp. of the sesame oil.
Broil the tuna:
Position an oven rack 5 to 6 inches from the broiler element and heat the broiler to high. Line the bottom of a broiler pan with foil and replace the perforated top part of the pan. Season the tuna on both sides with the cayenne and arrange on the broiler pan. Broil the tuna for 2 minutes, brush generously with the teriyaki sauce, and broil until the glaze sets, about 2 minutes longer.
Turn the tuna over with a spatula and broil for 2 minutes, brush generously with the teriyaki sauce, and broil until the tuna is pale pink in the center or to desired doneness (cut into a piece to check), about 2 minutes longer. Brush with any remaining teriyaki sauce.
Meanwhile, stir together the scallions, cilantro, vinegar, and the remaining 1/2 tsp. sesame oil. Transfer the tuna to serving plates, top with the scallion salad, and serve.
nutrition information (per serving):
based on four servings;
Photo: Scott Phillips