Cut the tuna sashimi into bite-sized pieces.
Dice the avocado and cucumber.
Finely chop the onion, soak it in cold water to remove the sharp flavor.
Make a sauce by mixing soy sauce, sesame oil, lemon juice, and wasabi.
Place rice in a bowl and top with all the ingredients.
Drizzle the sauce on top and finish with roasted seaweed flakes and sesame seeds.