Wash the cucumber and slice it into bite-sized pieces.
Finely chop the onion and green onion.
Drain the tuna using a strainer to remove the oil.
Place rice in a bowl.
Top the rice with cucumber, tuna, onion, and green onion.
Mix soy sauce, sesame oil, and ground sesame seeds to make the sauce.
Drizzle the sauce over the rice bowl.