Add the potatoes to a large pot of cold water. Season well with salt and bring to a boil. Simmer until tender, 15 to 20 minutes.
Meanwhile, add the pancetta and onions to a large skillet. Turn on the heat to medium-high, and saute until the pancetta is crisp and golden. Add the milk and season with salt and pepper. Heat until hot.
Drain the potatoes well in a colander. Use a food mill to rice the potatoes into the milk mixture. Fold together the potatoes with the hot milk, olive oil and chopped parsley.
Recipe courtesy Gabriele Corcos and Debi Mazar, Extra Virgin, 2011