Drain the canned tuna thoroughly and place it in a medium mixing bowl.
Add the Greek yogurt, sriracha, lemon juice, sliced green onions, sea salt, and black pepper to the tuna, mixing until well combined and creamy.
Julienne the cucumber into thin matchsticks and slice the avocado into thin wedges.
Place one sheet of nori on a bamboo rolling mat or a clean flat surface.
Wet your fingers slightly and spread half of the cooked sushi rice in a thin, even layer over the nori, leaving about an inch of space at the top edge.
Place half of the tuna mixture in a horizontal line across the center of the rice.
Layer half of the cucumber matchsticks and avocado wedges on top of the tuna.
Carefully roll the nori from the bottom up, using the mat to keep it tight, and seal the top edge with a tiny bit of water.
Repeat the process for the second roll, then use a sharp, damp knife to slice each roll into 6-8 pieces.