Start by washing well the potatoes, without removing their skin and then cut them into large cubes and rectangles. Then take a big pot and fill it with water and leave it to boil.
When the water is boiling pour in the potatoes, with some salt.
In the meanwhile cur the onion into rings and fry it together with the previously cut peppers, until it gets soft. Then remove the ingredients from the pan and place them in a big bowl.
When the potatoes are boiled, remove them from the water and pour them in the bowl with the peppers and the onion, then add the mayonnaise, the tuna, the parsley, some salt pepper and olive oil and mix well, until the mayonnaise has spread everywhere.
Cover the bowl with a cling wrap and place it in the fridge for about 60 minutes. Serve the potato salad when it is cold