In a large pot of boiling salted water add the spaghetti and cook until it is al dente. Meanwhile, in a large skillet set over moderately low heat, melt the butter, add the rosemary and cook it, stirring, for 1 to 2 minutes. As soon as the pasta is cooked, drain and add it to the pan. Add the cream, salt, pepper and nutmeg. Toss to coat and let reduce 1 to 2 minutes, gently stirring, or until lightly thickened. Add Parmesan to taste, toss to combine and transfer to serving dish. Sprinkle with parsley.