Peel the potatoes and cook them in a large pot in 5 litres of salted water until tender, about 10 minutes. When the potatoes are cool, cut them into little cubes and set them aside.
Heat a large frying pan over medium high heat and melt the butter. Add the onion and pancetta and cook until the onion is tender and the pancetta is browned. Add the potatoes, rosemary, olive oil, salt and pepper and continue cooking until the potatoes are browned.
You can cook the potatoes the night before and refrigerate them overnight.