Cook the spaghetti al dente (firm but not hard), in a large pot of boiling salted water according to package directions.
Meanwhile, fry the pancetta and onion over medium-low heat in a large, straight sided skillet until the pancetta is crispy and the onion has softened. Turn the heat off.
In a medium mixing bowl, whisk together the yolks, cream and grated cheese. When the spaghetti is ready drain and toss it into the skillet with the pancetta and onion. Quickly pour the egg mixture over the spaghetti and toss everything together well to combine. The residual heat from the skillet, onions and spaghetti will be enough to cook the eggs and make a creamy sauce. Direct heat from the stovetop will make an overcooked, scrambled egg, lumpy mess. Season the spaghetti with salt and pepper and serve immediately.