In a bowl, mix the diced fish, soy sauce and mayonnaise. Cut each nori sheet into 4 equal pieces.
Arrange the sheets in a muffin tin by pushing one down into each muffin hole to the bottom. In a bowl, mix cooked rice, salt and rice vinegar.
Divide the cooked rice into the seaweed cups. Put the fish on top of the rice. Sprinkle with sesame seeds. Bake 10 - 15 minutes in 400 F oven (or until fish is cooked to your liking). Sprinkle with green onions.