Cut fresh salmon and tuna into bite-sized cubes.
Dice avocado and cucumber into cubes.
Finely chop the onion and soak it in cold water to remove its sharp flavor.
In a bowl, make a sauce by mixing soy sauce, sesame oil, lemon juice, and sugar.
Place rice in a bowl and drizzle with the prepared sauce.
Top with the diced fish, avocado, cucumber, onion, and seaweed.
Add your desired toppings (flying fish roe, seaweed, sesame seeds, etc.).