Melt the butter in a sauce pan and wilt the onion in it, add the cream, and bring it to a low boil, stirring. Stir in the wine. Now blend a bit of the hot cream with the egg yolk and then whisk the yolk mixture into the cream. Gradually add the cheeses, a handful at a time, stirring and blending without allowing the sauce to a boil again. Keep it warm while you cook the fettucine. When noodles are done, drain and toss with the sauce till well coated. Serve with extra sauce on top. Serves 4.