Place one nori sheet horizontally on a bamboo rolling mat, with the rough side facing up.
Dip your fingertips in water and spread about ½ cup / 100 g (3.5 oz) of the sushi rice in a thin, even layer across the bottom three-quarters of the nori sheet.
Arrange one-quarter of the cucumber matchsticks in a horizontal line across the center of the rice. Ensure the cucumber extends to the edges of the nori.
Sprinkle ¼ teaspoon of the toasted sesame seeds over the rice. For the Cucumber Mint variation, first smear ¼ teaspoon of tahini across the rice and add a few torn mint leaves before adding the cucumber.
Place your thumbs underneath the bamboo mat and use your fingers to hold the fillings in place.
Fold the bottom edge of the mat up and over, tucking the edge of the nori just past the cucumber filling.
Lift the edge of the mat and continue to roll the nori forward, applying gentle but firm pressure to create a tight cylinder.
Shape the roll by gently pressing the top and sides with your fingers over the mat to create a subtle square shape.
Set the finished roll seam-side down on a cutting board.
Repeat the process to make the remaining three rolls.
Using a very sharp knife dipped lightly in water, slice each roll into 6 even pieces.
Serve immediately.
Keep a small bowl of water (tezu) nearby to wet your fingertips. This prevents the sticky sushi rice from clinging to your hands while spreading.
Don't overfill your rolls. A thin layer of rice and a neat line of fillings are key to a roll that holds together perfectly.