Cook the macaroni until al dente, about 10 minutes. Drain and place in a bowl. Stir in the sour cream and set aside.
Heat the milk in a saucepan until just beginning to bubble but not boiling. Reduce heat and add cheese, mustard, salt and pepper and nutmeg then stir until cheese has melted. Pour into the pasta and stir.
Heat up a small frypan and toast the bread crumbs and Parmesan until browning. Sprinkle over the pasta and serve.