In a small mixing bowl, combine the drained tuna, non-fat Greek yogurt, sriracha, lemon juice, sea salt, and black pepper until the mixture is creamy and well incorporated.
Place the cooked sushi rice in a separate bowl and gently fold in the rice vinegar using a spatula until evenly distributed.
Lay one nori sheet on a bamboo rolling mat or a clean flat surface with the rough side facing upward.
Lightly moisten your fingertips with water and spread half of the seasoned rice across the nori, leaving a 1-inch border at the top edge.
Arrange half of the tuna mixture, avocado slices, and julienned cucumber in a horizontal line across the center of the rice layer.
Carefully roll the nori from the bottom edge upward, keeping the filling tight, and use a small dab of water to seal the top seam.
Using a very sharp, damp knife, slice the roll into 8 even pieces and repeat the process for the second roll before serving.