Cook the rice according to package instructions until soft and fluffy. Let it rest for a few minutes and fluff gently.
Drain the tuna completely and place it in a bowl. Break it into small flakes using a fork.
Add mayonnaise, soy sauce, sesame oil, lemon juice, and black pepper to the tuna. Mix gently until creamy but still slightly chunky.
Chop cucumber into small pieces, shred the carrots, and slice green onions thin.
Add warm rice to a serving bowl and spread it evenly as the base.
Spoon the tuna mixture over the rice. Do not press it down too much.
Add chopped cucumber and shredded carrots around the tuna mixture.
Sprinkle green onions and sesame seeds on top.
Serve immediately while warm.