Poke
Cut the tuna into bite-sized pieces.
Slice the onion and cucumber thinly.
Remove the skin and pit from the avocado, then dice it.
In a bowl, make the dressing by combining soy sauce, sesame oil, and lemon juice.
Lightly toss the tuna with the dressing.
Place the tuna, vegetables, and avocado on top of the rice.
Sprinkle seaweed flakes and sesame seeds on top.