Dice the salmon into bite-sized cubes.
Thinly slice the onion and cucumber.
Dice the avocado.
In a bowl, mix together soy sauce, sesame oil, sugar, and lemon juice to make the sauce.
Gently toss the salmon and vegetables in the sauce.
Place the salmon and vegetables on top of rice.
Finish by sprinkling seaweed flakes and sesame seeds on top.