Rinse the sushi rice under cold water until the water runs clear, then cook it according to the package instructions. Once cooked, transfer to a large bowl and gently fold in rice vinegar if using, to season the rice. Allow it to cool slightly until it’s warm but manageable to handle.
Lay a bamboo sushi mat on your work surface and place a sheet of nori shiny side down on top. Using wet hands, spread a thin, even layer of rice over the nori, leaving about an inch at the top edge free to seal the roll.
Arrange strips of cucumber, julienned carrots, avocado, and bell pepper horizontally across the bottom third of the rice-covered nori. Feel the crunch and freshness of each vegetable as you lay them out carefully.
Using the bamboo mat, gently lift the edge closest to you and begin to roll, pressing firmly but gently to keep the fillings tight and compact. Continue rolling until the seam is sealed at the top edge of the nori.
Once rolled, use a sharp knife to slice the sushi roll into 6-8 even pieces. Wipe the knife with a damp cloth between cuts to achieve clean, smooth slices.
Arrange the sliced sushi on a plate, showcasing the vibrant colors and textures of the vegetables inside. Serve immediately with soy sauce, pickled ginger, or wasabi if desired.
