Mix the sesame oil, tamari sauce, ginger, garlic, onion, and lime juice. Pour this over the tuna steaks in a glass dish, cover, and marinade for 10 minutes to 2 hours in the refrigerator.
Bring the grill to medium-high heat. Remove tuna from the marinade and place on grill for 2 − 3 minutes each side. Remove and let sit for about a minute before serving.