Gather the ingredients. The Spruce Eats / Victoria Heydt. In a large zip-top bag, combine the soy sauce, chopped scallions, lemon juice, sesame oil, and fresh ginger. Swish everything around in the bag until it's well combined. The Spruce Eats / Victoria Heydt. Add the tuna steaks, turning to coat them completely with the marinade. Slice one side with 1/4-inch diagonals with 1/4-inch spaces in between. This will help you slice the tuna steak easier without mushing the meat. Soak the tuna in the marinade for 20 minutes. Follow the same procedure done with the searing, grilling the tuna at 45 seconds per side. Sprinkle both sides with a pinch of salt and several grinds of black pepper. When the grill pan is blazing hot, place the tuna steaks in the pan. You'll hear a nice sizzling sound. Lower the heat to medium high (the cast iron will retain the high heat so you won't see an immediate change in the cooking). Let the steaks sit, undisturbed, for 2 y9ZGC1E.