Cut the Ahi tuna in 1 inch pieces and place in a bowl with sesame seeds, soy sauce, chili flakes, and half of the chopped green onions. Let rest for 25 minutes. Place the thinly sliced onion in a bowl with 2 tbsp of vinegar and 2 tbsp water for 5 minutes.
Toast the buns and top with sliced red onions, sliced avocado, sliced cucumber, poke, extra sesame seeds, cilantro and green onions.