Rinse the sushi rice under cold running water until the water becomes mostly clear. Drain well to remove excess water. This helps remove extra starch and keeps the cooked rice from becoming too sticky.
Add the rinsed rice and water to a medium saucepan. Bring it to a boil, then reduce the heat to low. Cover and cook for 12–15 minutes, or until the water has been absorbed and the rice is tender. Remove the saucepan from the heat and let the rice rest, covered, for 10 minutes.
In a small bowl, mix the rice vinegar, sugar, and salt until dissolved. Transfer the cooked rice to a large bowl and gently fold in the vinegar mixture using a rice paddle or wooden spoon. Allow the rice to cool to room temperature before assembling the rolls.
Wash and prepare the cucumber, carrot, avocado, and red bell pepper. Cut them into thin, even strips so they fit neatly inside the sushi rolls.
Keep the avocado slices slightly thicker to give the rolls a creamy texture. Arrange all the prepared vegetables near your rolling station for easy assembly.
Place a bamboo sushi mat on a clean work surface. Lay one sheet of nori on the mat with its shiny side facing down and the rough side facing up.
Wet your fingers with a little water to prevent the rice from sticking. Spread about one-quarter of the prepared sushi rice evenly over the nori, leaving a small border along the top edge.
Arrange a few strips of cucumber, carrot, avocado, and red bell pepper horizontally across the center of the rice. Avoid adding too much filling, as this can make the rolls difficult to close.
Lift the edge of the bamboo mat closest to you and begin rolling the nori over the filling. Use your fingers to keep the vegetables in place while gently tucking the roll inward.
Continue rolling forward, applying gentle and even pressure with the bamboo mat until the sushi roll is completely sealed. Lightly press along the roll to help it hold its shape.
Repeat the process with the remaining nori sheets, rice, and vegetables.
Use a sharp knife to cut each sushi roll into 6–8 even pieces. Wipe the knife with a damp cloth between cuts to keep the slices clean and prevent the rice from sticking.
Arrange the vegetarian sushi pieces on a serving plate. Sprinkle with toasted sesame seeds if desired and serve with soy sauce, pickled ginger, and wasabi.
