Finely chop the onion and pancetta and fry in the butter until golden. The onions should be really soft.
In a preheated bowl, mix the eggs with cheese and a lot of pepper, add the spaghetti to the bowl and mix quickly with the onions and pancetta.
If you want to prepare the dish with cream, mix the cream (150 ml per person) with the eggs and heat the whole thing in a water bath before pulling it into the hot noodles.