Rinse the sushi rice under cold water until the water runs clear.
Cook the rice according to package instructions.
In a small saucepan, heat the rice vinegar, sugar, and salt until dissolved. Let it cool.
Once the rice is cooked, transfer to a bowl and fold in the vinegar mixture. Let cool to room temperature.
Lay a sheet of nori on a sushi mat, shiny side down.
Spread a thin layer of sushi rice evenly over the nori, leaving a ½ inch border at the top.
Arrange cucumber, avocado, and sushi-grade fish (if using) near the bottom of the nori.
Using the sushi mat, roll the nori tightly from the bottom up, applying gentle pressure.
Sprinkle tobiko over the roll for an even coating.
Use a sharp knife to cut the roll into 6-8 pieces, cleaning the knife between cuts.
Serve with soy sauce, wasabi, and pickled ginger.