Season the sushi rice with the rice vinegar, fanning and stirring until room temperature.
On a rolling mat, place one sheet of nori with the rough side facing upwards.
Wet your hands and grab a handful of rice and place it on the nori. Spread the rice evenly throughout the nori without mashing the rice down. Season rice with a pinch of sesame seeds, if using, then flip it over so the nori is facing upwards.
Arrange, in a horizontal row 1 inch (2 cm) from the bottom, the crab followed by a row of avocado and a row of cucumber.
Grabbing both nori and the mat, roll the mat over the filling so the extra space at the bottom touches the other side, squeezing down to make a nice tight roll. Squeeze down along the way to keep the roll from holding its shape.
Transfer the roll onto a cutting board. Rub a knife on a damp paper towel before slicing the roll into six equal portions.