Cut the tuna sashimi into bite-sized pieces.
Thinly slice the onion and cucumber.
Dice the avocado.
Place rice in a bowl.
Top the rice with the prepared tuna, onion, cucumber, avocado, and cherry tomatoes.
Mix soy sauce, sesame oil, lemon juice, and wasabi to make the sauce.
Drizzle the sauce over the ingredients and finish with seaweed flakes and sesame seeds.