In a bowl, whisk together the first 7 ingredients to make a marinade. Towel dry tuna steaks and place in a glass or ceramic dish. Pour marinade over the fish, turning to coat well throughout. Cover tightly and chill several hours to overnight; turn fish over at least once in the middle of marinade time.
Heat 3 tablespoons of olive oil in a large heavy skillet until oil is smoking. Sear bluefin steaks 1 1/2 minutes per side on high heat, taking care to flip carefully with a flat, steel spatula. (If your tuna is less than 1 1/2 inches thick, sear for only 1 minute per side.)
Transfer seared bluefin to a large cutting board and let rest/cool 10 minutes. Use a very sharp knife to slice thinly across the grain.
Serve chilled or at room temp, over your favorite salad greens. Sprinkle with sesame seeds and cilantro or green onions, if desired. Drizzle with Sriracha mayo for an added depth of flavor!