Whisk tamari, sesame oil, mirin, honey, vinegar, ginger, green onions, and garlic together in a bowl until marinade is evenly mixed. Place tuna steaks in the marinade and refrigerate for at least 1 hour.
Spread sesame seeds onto a plate. Remove tuna from marinade and discard marinade. Press tuna into sesame seeds until evenly coated.
Heat olive oil in a cast iron skillet over high heat until very hot; cook tuna in the hot oil until cooked to desired doneness, 30 seconds to 5 minutes per side.
Notes
Grilling option: do not coat tuna with sesame seeds! Spray grill with non-stick cooking spray for grills. Heat grill until very hot. Grill tuna on each side for 2 minutes (for medium rare).
Recipe by Make Mine Gluten Free at https://www.makemineglutenfree.com/asian-sesame-seared-or-grilled-tuna-gluten-free/