Cook sushi rice according to package instructions. In a small bowl, combine vinegar and sugar. Once the rice has slightly cooled, gradually pour and fold the mixture into the rice. Place plastic wrap in a round dish (around 6 in diameter) with straight edges. Spread and pack rice to form a disc. Cover with plastic wrap and chill in the fridge until slightly hardened.
In a small dish, beat egg and a little water together. In another dish add flour and panko breadcrumbs in another. Coat the rice patty in flour, dip in the egg mixture, and then into the panko crumbs. Fry on all sides until golden brown and crispy.
Prep toppings: Thinly slice salmon, tuna, and avocado. Set aside. In a small bowl combine mayo and sriracha. Slice thin strips of green onion lengthwise. Place in a small bowl of ice water to curl. Dry on paper towel.
To assemble, spread spicy mayo on the fried rice patty. Top with sliced avocado. Then place alternating salmon and tuna slice in a circular pattern. Add green onion curls in the center and sprinkle black sesame seeds on top.
