Combine the soy sauce, sugar, sherry. ginger, garlic, vegetable oil, sesame oil and black pepper in a large resealable bag. Add the tuna steaks and marinate for 15 minutes. When ready to cook, remove the tuna and reserve the marinade.
Preheat a cast iron grill over medium high heat. Brush a little oil. Grill the tuna 1 to 2 minutes per side. In a small saucepan bring the reserved marinade to a boil until slightly thickened. Sliced the tuna and arrange them on a platter. Pour some of the sauce and garnish with sesame seeds and green onions.
Recipe by The Gluten Free at https://www.theglutenfree.com/gluten-free-grilled-teriyaki-tuna/