In a bowl, whisk together the marinade ingredients. This can be done in advance. Cover and chill until ready to use.
Thoroughly dry ahi tuna steaks with paper towels and place in a glass or ceramic dish. Pour marinade over the fish, turning to coat well throughout. Cover tightly and chill several hours, or up to overnight. Turn fish over at least once in the middle of marinade time.
Heat 3 TB oil in a large heavy skillet until oil is smoking. Sear ahi 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 ahi to a large cutting board and let rest/cool 10 minutes. Use a very sharp (serrated works well) knife to slice thinly across the grain.
Serve chilled or at room temp. Sprinkle with sesame seeds and cilantro or green onions, if desired. Drizzle with Sriracha mayo if desired.
Do not marinade longer than overnight, as the acid from the lemon can affect fish texture if soaked too long.
Use a heavy, large aluminum or stainless steel skillet; forgo nonstick pans here. We've used our faithful All Clad stainless steel skillet for many, many years, and it continues to serve us well. It's a heavy duty work horse in the kitchen and is meant to last a lifetime.
Ahi may continue to cook a bit more while resting on the cutting board, so don't overcook it in the pan.
Seared ahi is best eaten on same day. However, leftovers can be tightly wrapped and chilled for up to 1 day.
To keep dish gluten-free, double check that your soy sauce is labeled gluten-free. Our favorite gluten-free soy sauce is Tamari.
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