Step 1

Blend vinegar, soy sauce, ginger, garlic, wasabi (or horseradish), sugar, and green onion. Taste sauce, adjust seasoning if necessary (adding more soy, sugar, ginger etc if preferred). Marinate tuna in blended sauce for about 10 minutes. Sprinkle sesame seeds on top of tuna at end of marinade.

Step 2

Heat oil in a heavy bottom pan (like a cast-iron pan) on high, until oil is very hot. Reserving marinade for later, sear tuna for 30-45 seconds per side, then remove from pan.  (For tuna loins with 3 sides, sear all 3 sides for about 20 seconds each.)

Step 3

Reduce heat to medium and allow pan to cool slightly. Add marinade to pan with lemon juice, and stir with a wooden spoon. If sauce is too thick for your liking, add more lemon juice. Remove sauce from heat.For steaks: Slice tuna in thin strips. If desired, leave about 1/2" at one end of the tuna steak uncut, and serve fanned on a plate with ginger soy pan sauce and extra green onion to garnish.For loins: Thinly slice the tuna and serve fanned on a plate with ginger soy pan sauce and extra green onion to garnish.