Rinse the sushi rice under cold water until clear, then cook according to package instructions and fold in the rice vinegar while warm.
Slice the fresh yellowfin tuna into long, thin strips approximately half an inch thick.
Julienne the cucumber into thin matchsticks and slice the avocado into long wedges.
Place the nori sheet on a bamboo rolling mat, then spread the seasoned rice evenly over the bottom two-thirds of the sheet.
Arrange the tuna strips, cucumber matchsticks, and avocado wedges in a horizontal line across the center of the rice.
Roll the nori tightly from the bottom using the mat to guide it, sealing the top edge with a small drop of water.
Slice the roll into eight even pieces using a sharp, damp knife to ensure clean cuts.
Serve the rolls immediately with coconut aminos, wasabi, pickled ginger, and a sprinkle of toasted sesame seeds.