Warm a heavy pot over medium-low heat. Add the butter, olive oil, and pancetta; cook, stirring, until the fat renders and the pancetta edges turn golden, 5–7 minutes.
Stir in the onion, carrot, and celery with a pinch of kosher salt. Cook gently, stirring occasionally, until very soft and sweet but not browned, 10–12 minutes.
Increase heat to medium. Add the ground beef and break it up with a spoon; cook until no longer pink and lightly browned in spots, 8–10 minutes. Season with the black pepper.
Add the milk and nutmeg; simmer, stirring, until the milk has mostly evaporated and the meat looks creamy and glazed, 8–10 minutes.
Pour in the white wine; simmer until nearly dry and no raw alcohol aroma remains, 8–10 minutes.
Near the end of the simmer, bring a large pot of water to a rolling boil and cook the rigatoni until al dente, 10–12 minutes. Reserve 1 cup pasta water, then drain.
Taste the ragù and adjust seasoning. Add the drained rigatoni to the pot with enough ragù to coat (about 4 cups) and a splash of reserved pasta water; toss over medium heat until the sauce clings to the pasta, 1–2 minutes.