Cook the rice: Rinse 1.5 cups of sushi rice in cold water until it runs mostly clear.
Cook according to package directions with the proper water ratio. Let it stand covered for 10 minutes after cooking to finish steaming.
Season the rice: In a small saucepan, gently warm 3 tablespoons rice vinegar, 1 tablespoon sugar, and 1 teaspoon salt until dissolved. Do not boil.
Transfer the hot rice to a wide bowl and fold in the vinegar mixture with a spatula. Fan the rice or let it cool until just slightly warm. Cool rice is easier to handle and helps prevent soggy nori.
Prep the vegetables: Slice cucumber, carrot, and bell pepper into thin matchsticks about 3-4 inches long. Slice avocado last to keep it from browning.
Keep everything in neat piles so rolling goes quickly.
Set up your station: Place the bamboo mat on a cutting board. Put a nori sheet, shiny side down, on the mat with the lines running horizontally. Keep a small bowl of water nearby to wet your fingers.
Spread the rice: Wet your fingertips and spread a thin, even layer of rice over about two-thirds of the nori, leaving the top third uncovered.
Don’t mash the rice—light pressure keeps it fluffy. Too much rice makes rolling hard and can split the nori.
Add fillings: Arrange a narrow line of cucumber, carrot, bell pepper, avocado, and a sprinkle of green onion along the bottom edge of the rice. Keep it tidy and not too thick. Add a light sprinkle of sesame seeds.
Roll it up: Lift the mat edge closest to you and roll over the fillings, tucking tightly to form a firm cylinder.
Pull the mat back as you roll forward so it doesn’t get trapped inside. Wet your fingertip and dab the bare strip of nori at the top to seal the roll.
Rest and repeat: Let the roll seam-side down while you make the others. This helps it seal fully.
You should get about 5–6 rolls from this batch.
Slice cleanly: Use a very sharp knife. Wipe the blade with a damp cloth or lightly oil it between cuts. Slice each roll in half, then into 6–8 pieces. Sawing crushes the roll; use gentle, confident strokes.
Serve: Arrange on a plate with soy sauce or tamari for dipping, plus pickled ginger and a dab of wasabi if you like.