In a small bowl, whisk together soy sauce, sesame oil, rice vinegar, minced garlic, and grated ginger.

Place the Ahi tuna steaks in a shallow dish and pour the marinade over them. Let marinate for 10 minutes.

Heat a skillet over high heat. Remove tuna from marinade and sear for 1-2 minutes on each side, until the outside is browned but the inside remains rare.

Slice the seared tuna against the grain and serve garnished with toasted sesame seeds and chopped green onions.