In a small bowl, gently fold the rice vinegar and sea salt into the warm cooked sushi rice until well combined and glossy.
Slice the sashimi-grade ahi tuna into long, thin strips approximately half an inch thick.
Julienne the cucumber into thin matchsticks and slice the avocado into thin wedges.
Place one nori sheet on a bamboo rolling mat, then spread half of the seasoned rice evenly over the bottom two-thirds of the sheet using damp fingers.
Layer half of the tuna, cucumber, and avocado in a horizontal line across the center of the rice.
Carefully roll the nori from the bottom up, using the mat to apply firm pressure until a tight cylinder is formed.
Repeat the process with the second nori sheet and the remaining ingredients.
Use a very sharp, slightly damp knife to slice each roll into 6 to 8 pieces.
Sprinkle with sesame seeds and serve immediately with coconut aminos mixed with sriracha for dipping.