Rinse sushi rice under cold water until the water runs mostly clear, then cook according to the package instructions. While the rice is still warm, season with rice vinegar, sugar, and salt to taste and gently fold to combine.

Slice the cucumber into thin, even strips and slice the avocado. Flake or chop the cooked shrimp or crab so it’s easy to distribute along the roll. Set everything within easy reach.

Place a bamboo mat covered with plastic wrap on your workspace. Lay a nori sheet shiny side down on the mat. Wet your hands (this prevents sticking) and have a small bowl of water handy.

Spread about 1 cup of sushi rice over two-thirds of the nori sheet, pressing gently to form an even layer. Add a line of shrimp or crab, cucumber strips, and avocado down the center of the rice. Drizzle with sriracha.

Use the bamboo mat to roll the nori over the filling, tucking the edge tightly and rolling away from you. Apply gentle, even pressure to shape a firm cylinder. Let it rest a minute to set.

Dip a sharp knife in water and slice the roll into eight pieces, wiping the blade between cuts for clean edges. Sprinkle with toasted sesame seeds.