Cut sashimi-grade tuna into bite-sized cubes.
Dice avocado, cucumber, and onion.
Place tuna, avocado, cucumber, and onion in a bowl.
Add soy sauce, sesame oil, lemon juice, sugar, and Cheongyang chili pepper. Mix well.
Spoon the mixed tuna and vegetables over rice.
Top with seaweed, sesame seeds, scallions, or other desired toppings to finish.