Peel the onion, cut it in half and finely slice it.
Cut the avocado in half, remove the core, scoop the pulp and cut it in chunks.
Wash the lettuce leaves thoroughly, cut the middle of the leaves out and then cut the rest in small chunks.
Cut the cucumber in quarters (length-wise) and then finely slice them.
Cut a few small bits/slices of the lemon half. Squeeze the rest and keep the juice.
Drain the tuna can and put the content in a salad bowl.
Add all the vegetables, the lemon bits and the juice, the salt and mix thoroughly.
Enjoy!