Whisk the egg yolk and cream together in a small jug. Pan fry the pork fillets on high in butter until browned and firm. Remove from pan and keep warm. Pan fry mushrooms until browned. Pour in wine and scrap the bottom of the pan. Allow to boil and reduce then turn down the heat to medium. Spoon in mustard. Stir well. Pour in cream and egg yolk mixture and stir until all combined. Allow the sauce to thicken slightly. Serve pork fillets with your choice of vegetables and spoon over the sauce. It is that simple!