Drain the canned tuna thoroughly to remove excess liquid and place it in a medium mixing bowl.
Combine the tuna with the Greek yogurt, Dijon mustard, sea salt, and black pepper, stirring until the mixture is creamy and well-incorporated.
Lay one nori seaweed sheet flat on a bamboo rolling mat or a clean cutting board with the rough side facing up.
Gently spread half of the cooked white rice over the bottom two-thirds of the nori sheet, leaving the top edge clear.
Place half of the tuna mixture in a horizontal line across the center of the rice, then layer on half of the avocado slices, cucumber matchsticks, and shredded carrots.
Starting from the bottom, tightly roll the nori over the filling, using a small amount of water to dampen the top edge of the seaweed to seal the roll.
Repeat the process with the second nori sheet and the remaining ingredients.
Use a sharp, slightly damp knife to slice each roll into six to eight even pieces and serve immediately with coconut aminos for dipping.