Cover the sheet of nori in a thin even layer of rice, filling all the way out to the edges. Optionally, top the rice with some black sesame seeds. (click for guide)
Flip the sheet of nori over so the rice is face down on a bamboo rolling mat that is wrapped in plastic. (click for guide)
Arrange the fillings (umami tuna, cucumber, and avocado) in even lines, 1 inch from the edge along the longer side of the nori sheet. (click for guide)
Place the plastic-wrapped rolling mat over the top of the roll and press the roll into a consistent shape. (click for guide)
Place the roll on a cutting board and grab your sharpest knife. Run the knife under water for a second, then shake off any excess. Cut the roll into 8 equal pieces using a gentile sawing motion with the knife, running the knife under the water between every 2-3 cuts to prevent sticking. (click for guide)