Rinse 1 cup of short-grain white rice under cold water until the water runs clear. Cook the rice according to package instructions, then let cool to room temperature.

While the rice cools, slice cucumber into thin strips and dice the avocado. Break imitation crab into manageable pieces.

Lay a bamboo sushi mat on a flat surface and place a sheet of nori, shiny side down. Mix water with rice vinegar to keep hands wet.

With wet hands, spread the cooled sushi rice over the nori, leaving about an inch at the top edge.

Horizontally place imitation crab, cucumber strips, and avocado slices across the center of the rice.

Starting from the bottom edge of the mat, carefully lift and fold the mat over the filling. Roll forward gently and apply slight pressure.

To slice the roll, use a sharp knife dipped in water to cut the roll into 6 to 8 equal pieces.

Optionally, sprinkle tempura flakes on top of sliced rolls and serve with soy sauce, wasabi, and pickled ginger.