Pat the tuna steaks dry with paper towels and season with salt and pepper.
Brush each side of the tuna with sesame oil and press sesame seeds onto both sides until evenly coated.
In a small bowl, combine soy sauce, mirin, rice vinegar, ginger, and garlic; whisk until mixed.
Heat a skillet over medium-high heat and sear the tuna for about 1-2 minutes on each side for rare to medium-rare, until golden brown.
Remove from heat, allow to rest for 2 minutes, then slice against the grain into thin pieces. Drizzle with soy-mirin sauce and garnish with green onions and sesame seeds.
Notes
Use a very sharp knife for clean cuts when slicing the tuna and ensure your pan is hot enough to sear without steaming the tuna.