Wash the tomatoes and cucumbers thoroughly and cut them into bite-sized pieces.
Thinly slice the onion.
Drain the oil from the canned tuna and prepare it.
In a bowl, mix the tomatoes, cucumbers, onions, and tuna with soy sauce, sesame oil, vinegar, and sesame seeds.
Place in a serving bowl and add seaweed flakes or a squeeze of lemon juice if desired.