In a small bowl, gently fold the rice vinegar and sea salt into the warm cooked sushi rice until well combined and glossy.
Place the nori sheet on a bamboo rolling mat, shiny side down.
Moisten your fingers with water and spread the seasoned rice evenly over the bottom two-thirds of the nori sheet, leaving the top edge clear.
Slice the sashimi-grade salmon and avocado into long, thin strips.
Peel and slice the cucumber into thin matchsticks, removing the watery seeds.
Layer the salmon, avocado, and cucumber matchsticks in a horizontal line across the center of the rice.
Using the bamboo mat, firmly roll the nori from the bottom up, tucking the ingredients tightly as you go.
Lightly dampen the top edge of the nori with a drop of water to seal the roll.
Use a very sharp, wet knife to slice the roll into 6-8 even pieces.
Serve immediately with coconut aminos for dipping and pickled ginger on the side.