Wash the potatoes and boil without peeling for 30 minutes. Drain and let cool a bit before you peel. With a fork mash the potatoes and season them with salt and white pepper.
Add the tuna to the mashed potatoes and mix. Add the chopped onion as well and mix well together. Set aside.
In a bowl put 3/4 of the mayonnaise and add the lemon juice. Whisk together using a fork. Don't worry if it's a bit watery, the potatoes will absorb. Add the olive oil to the mayo and more lemon according to taste. If it's too sour for your taste you can add more mayonnaise to even it out.
Pour the mayo mix into the potato and tuna and mix until it's all well blended. Last add the chopped parsley.
Serving suggestion: lay the potato-tuna salad in a platter and with the back of a spoon smooth it out. Then spread extra mayonnaise mixture on top and garnish with some parsley leaves. Chill in the fridge for at least a couple of hours before serving.
You can also add sweetcorn or spring onions or even pickled cucumber if you like.