Place rice in a bowl.
Tear the K-rab (imitation crab) along the grain.
Dice cucumber, avocado, and carrot into bite-sized pieces.
Thinly slice the onion and soak it in cold water to remove the pungency.
Mix mayonnaise, mustard, lemon juice, and salt to make the sauce.
Top the rice with K-rab, cucumber, avocado, carrot, and onion.
Drizzle the sauce and finish with seaweed flakes and sesame seeds.