Preheat oven to 400°F and season the salmon fillet with sea salt and black pepper.
Bake the salmon for 12-15 minutes until it flakes easily with a fork, then set aside to cool slightly.
Cook the brown rice according to package instructions and toss with rice vinegar while still warm.
Slice the avocado into thin wedges and cut the cucumber into long, thin matchsticks.
Place the nori sheet on a bamboo rolling mat and spread the seasoned brown rice evenly over the bottom two-thirds.
Layer the flaked salmon, avocado wedges, and cucumber matchsticks across the center of the rice.
Roll the nori tightly from the bottom edge, using a small dab of water to seal the top seam.
Slice the roll into 8 even pieces using a sharp, wet knife and sprinkle with sesame seeds.
Serve the rolls immediately with coconut aminos for dipping.