Drain the oil from the tuna using a strainer and set aside.
Finely chop the onion, soak it in cold water to remove the spiciness, then drain well.
Slice the cucumber thinly, and prepare the corn as well.
In a bowl, combine the tuna, onion, cucumber, corn, mayonnaise, salt, and pepper, and mix well.
Serve on a plate and sprinkle with parsley or pepper according to your preference.