Rinse the sushi rice in a sieve under cold water until the water runs clear.
Cook the rice according to package instructions.
In a small saucepan, combine rice vinegar, sugar, and salt. Heat until the sugar dissolves.
Once the rice is cooked, transfer it to a large bowl and fold in the vinegar mixture. Let it cool.
Slice the raw fish into small cubes.
In a separate bowl, mix the soy sauce, sesame seeds, sriracha sauce, and mayonnaise.
To assemble the sushi stack, press a layer of rice into a small bowl, then add a layer of fish, avocado, and cucumber.
Repeat the layers to reach the desired height, then invert the bowl onto a plate to create the stack.
Drizzle the sauce mixture over the stack and garnish with nori pieces.
Serve immediately and enjoy!