Rinse 2 cups of sushi rice under cold water until it runs clear, then cook according to package instructions. Transfer to a large bowl and season with rice vinegar, sugar, and salt while warm. Allow to cool to room temperature.

Slice the cucumber and avocado into thin strips, about ¼ inch thick, and set aside.

On a flat workspace, lay out a bamboo sushi mat covered with plastic wrap. Place a sheet of nori, shiny side down, on the mat.

With wet hands, grab about 1 cup of sushi rice and spread it over the nori, covering two-thirds of the sheet, leaving the top third bare.

In the center of the rice layer, lay out the shrimp or crab, followed by cucumber and avocado strips. Drizzle sriracha sauce if desired.

Lift the edge of the bamboo mat and roll the nori away from you, tucking the fillings in as you go. Seal the roll with a drop of water.

Slice the roll into eight equal pieces with a sharp, wet knife and serve with soy sauce and extra sriracha.