Marinate the Tuna: In a bowl, mix soy sauce, sesame oil, rice vinegar, ginger, and garlic.
Place tuna steaks in the marinade and coat well. Let sit for 15–20 minutes.
Prep the Salad: Toss salad greens with olive oil, lemon juice, salt, and pepper. Set aside.
Coat with Sesame Seeds: Remove tuna from the marinade. Pat dry slightly. Press sesame seeds onto both sides of each steak.
Sear the Tuna: Heat olive oil in a pan over high heat.
Sear tuna steaks for about 1–2 minutes per side (outside should be golden, inside still pink).
Serve: Slice tuna and plate over the salad greens. Drizzle with any remaining marinade (boil it first if using).