Dice the tuna and avocado into bite-sized cubes.
Thinly slice the onion and cucumber.
In a bowl, mix soy sauce, vinegar, lemon juice, sugar, and sesame oil to make the sauce.
Place rice in a serving bowl and add the prepared ingredients on top.
Drizzle the sour soy sauce over the ingredients.
Finish by topping with seaweed, sesame seeds, and other desired toppings.