Soak the unpeeled potatoes in water for 30 minutes.
Wash them well, using a brush to remove the dirt.
Cook in water, salt and a little vinegar, until they are very soft (don't let them cook too much, they can't get too soft).
Drain and wait to cool.
If you want to poke holes in the potatoes with a thin toothpick for the seasonings to penetrate, being careful not to deform them.
In a pan put the olive oil, the oil, the pepperoni, the grated onion, the crushed garlic and the parsley.
Put the potatoes on top.
Sprinkle with salt and add the olives.
Sauté well for about 20 minutes (no need to stir, just shake the covered pan to mix the seasonings).
Allow to cool well and transfer everything to a large jar or platter with a lid.
If necessary, add more oil, as the liquid should completely cover the potatoes.
Refrigerate for at least 2 days before serving.