Wash the cucumber thoroughly and slice it diagonally.
Thinly slice the onion and soak it in cold water to remove the pungent taste.
Drain the oil from the tuna using a strainer.
Place the cucumber, onion, and tuna in a bowl.
Add mayonnaise, salt, pepper, and lemon juice, then mix well.
Finish by sprinkling chopped chives or parsley and other herbs on top.