- Combine the soy sauce, mirin, vinegar, ginger, garlic and brown sugar in a bowl and whisk together well. Add the sesame oil and blend. 
- Place the tuna and marinade to a large zip-top bag and refrigerate for 1 to 2 hours. 
- Prepare a hot grill, or heat a heavy cast-iron or nonstick skillet over medium-high heat. Remove the fish from the marinade. Reserve marinade.  Cook the steaks for 2 to 4 minutes on each side depending on thickness of tuna.   Remove tuna from pan to plates.  Return marinade to hot pan til it bubbles.  Drizzle hot sauce over tuna.