Cut fresh salmon or tuna into bite-sized cubes.
Dice vegetables such as cucumber, avocado, and onion.
Mix soy sauce, sesame oil, mirin, lemon juice, and sugar to make a sauce.
Add the sauce to the diced fish and lightly toss.
Place the seasoned fish and vegetables evenly on top of rice.
Finish by adding toppings like seaweed, sesame seeds, or mayonnaise as desired.