Rinse the sushi rice under cold water until clear, then cook according to package instructions. Once cooked, fold in the rice vinegar and sea salt while still warm, then set aside to cool.
If using raw sushi-grade salmon, slice into long, thin strips. If using cooked salmon, lightly sear the fillet in a pan until just opaque and flake into large chunks.
Julienne the cucumber into thin matchsticks and finely shred the carrots.
In a small bowl, whisk together the Greek yogurt and sriracha to create a high-protein spicy sauce.
Place one nori sheet on a bamboo rolling mat or a clean surface. Spread half of the seasoned rice in a thin layer over the nori, leaving the top inch clear.
Arrange half of the salmon, cucumber, and carrots in a horizontal line across the center of the rice.
Drizzle with half of the spicy yogurt sauce and sprinkle with black pepper.
Roll the nori tightly from the bottom up, using a dab of water to seal the top edge. Repeat for the second roll.
Use a very sharp, damp knife to slice each roll into 6-8 pieces and serve immediately.