Place sesame seeds in a sauté pan and toast over low heat, tossing often. Do not leave side of it, as they will become golden brown very quickly, less than 5 minutes.
Using paper towels pat dry the tuna steaks. Brush both sides with olive oil and sprinkle it with salt and pepper.
Add 1-tablespoon olive oil (1 tablespoon per steak) to a sauté pan and preheat it in medium high heat. Once the oil sizzles, place the tuna steak and sear each side it for 2 minutes. Repeat the same process for the second steak, if needed. The tuna should be seared on the outside and raw inside.
Set aside and let it cool. Slice each steak into large bite-size cubes.
Make the dressing:
Whisk olive oil, lime juice, lime zest, wasabi powder, soy sauce, Tabasco, salt and pepper in a small glass bowl. Toss the avocados in the dressing and mix to coat.
Spread greens on individual plates; arrange the thick bites of seared tuna and red onions on each plate.
Drizzle it with the dressing and sprinkle it with scallions and toasted sesame seeds.