Peel and cut the potatos in medium sized pieces (to be able to easily eat). Place them in a pot with water (they have to be submerged) and cook over low medium heat until completely cooked, for about 15 minutes.
Place a pan over medium heat with some oil. Chop the onion in small cubes and put it on the pan. Cook for 4 to 5 minutes until it is trnsparent and a little golden. Once ready, take off the heat.
Drain the pickles and cut them into slices. Once the potatos are totally cooked, drain them and let them cool for at least 5 minutes.
Put the potatos in a big bowl along with the onion, pickles, mayonnaise and mustard. Season with salt and pepper and mix until it all integrates, but careful to not overmix so the potatos stay in one piece.
Refrigerate for at least 10 minutes or even better overnight to have the flavors better.