Boil the pasta according to the package directions.
Place the pancetta in a large pan. Heat it over medium-high heat for about five minutes, stirring occasionally.
Add the red onion and cook it with the pancetta until the pancetta is lightly browned.
Add the butter to the pan and stir until it's melted. Add the cream and stir. Once the cream is steaming hot (after a minute or so), add the cheese and black pepper. Stir until the cheese is melted.
Add the cooked pasta to the pan and stir to coat it in the sauce. Garnish with the parsley for serving.
Notes
Storage: This pasta is best when freshly made, but leftovers will keep in the refrigerator for a few days. The sauce won't be as creamy once it's been chilled, so add a few splashes of milk or cream when you reheat it. I think it works best to reheat the pasta on the stove. I don't recommend freezing the leftovers.