Heat the oil in a large frying pan over medium heat.
Bring a large pot of water to the boil.
Sauté the onions until they just begin to soften, adding a pinch of salt to prevent them browning.
Add the ground beef, stirring and breaking up into small pieces.
Cook, stirring almost constantly, until the meat is almost browned.
Stir in the mushrooms and continue to cook until the meat is fully browned and the mushrooms have softened a bit.
Meanwhile, cook the macaroni in boiling salted water according to package directions.
Stir the beef stock into the meat mixture in the frying pan.
Add the mustard, paprika and some freshly ground pepper (to taste).
Stir together well and let the mixture bubble away for a few minutes, stirring occasionally.
Add the crème fraîche or sour cream and heat the mixture through.
Stir in the nutmeg.
Drain the macaroni and add it to the beef mixture in the frying pan.
Taste for seasoning and add salt and pepper if necessary.
Serve on warmed plates or bowls.