Cut the fish into cubes. In a large bowl, combine the fish, onions, ginger, chili flakes, soy sauce, sesame oil and salt; mix lightly. Chill for a couple of hours then add the chopped avocado and chill another couple of hours. Sprinkle with sesame seeds.
When we made ours, we used a ring mold and pressed in some of the sushi rice into the bottom (about 1/2" of rice) then topped with some of the poke and then carefully removed the mold onto each plate.