Melt the butter and sauté the onions with the lid closed and over low heat for about 20 minutes. Add a total of 4 tablespoons of water, do not let the onions turn brown.
Stir in the crème fraîche, salt, pepper, nutmeg and parsley. Cook the pasta in salted water until al dente, drain and add to the sauce. Mix well. Serve with freshly grated parmesan.