Slice the sashimi-grade ahi tuna into long, uniform 0.5-inch thick strips.
Julienne the cucumber into thin, crunchy matchsticks and slice the avocado into thin wedges.
In a small glass bowl, gently fold the rice vinegar into the warm cooked sushi rice until well combined.
Place the nori sheet on a bamboo rolling mat and spread the seasoned rice evenly over the bottom two-thirds of the sheet using damp fingers.
Layer the tuna strips, cucumber matchsticks, and avocado wedges horizontally across the center of the rice section.
Using the mat as a guide, roll the nori tightly from the bottom up, applying gentle pressure to ensure a firm roll.
Lightly dampen the top edge of the nori with water to seal the roll completely.
Use a very sharp, damp knife to slice the roll into 8 even pieces, wiping the blade between cuts for a clean finish.
Garnish with toasted sesame seeds and serve alongside tamari, wasabi, fresh ginger, and a bright squeeze of lime.