Prepare the sushi rice by folding in the rice vinegar and a pinch of sea salt while the rice is still warm.
Slice the sashimi-grade tuna into long, thin strips and julienne the cucumber and avocado into matchsticks.
In a small bowl, whisk together the Greek yogurt, sriracha, and black pepper to create a clean, zesty spicy sauce.
Place the nori sheet on a bamboo rolling mat and spread the rice evenly across the surface, leaving a one-inch border at the top.
Layer the tuna strips, cucumber, and avocado across the center of the rice.
Drizzle half of the spicy yogurt sauce over the fillings.
Roll the nori tightly using the bamboo mat, applying gentle pressure to ensure the roll is firm.
Using a sharp, wet knife, slice the roll into 8 even pieces.
Serve immediately with coconut aminos and the remaining spicy sauce on the side.