Wash the cucumber thoroughly and slice thinly.
Julienne the onion finely.
Sprinkle salt over the cucumber and onion, let them sit for 10 minutes, then remove the moisture.
Drain the oil from the tuna using a strainer.
In a bowl, combine the cucumber, onion, and tuna, then mix in mayonnaise and pepper.
Plate the salad and drizzle with lemon juice if desired.