In a saucepan, saute mushrooms in butter until softened. Stir in flour and 1/8 teaspoon pepper until blended. Gradually add the cream. Bring to a boil; cook and stir until thickened, about 2 minutes. Remove from the heat and set aside., In a bowl, combine the egg yolk, onion, 2 tablespoons mushroom sauce, salt and remaining 1/8 teaspoon pepper. Crumble beef over mixture and mix lightly but thoroughly. Shape into 2 loaves. Separate crescent dough into 2 rectangles on a baking sheet. Seal perforations. Place a meat loaf on each rectangle. Bring dough edges together and pinch to seal. If desired, brush with egg wash. Bake at 350° until golden brown and a thermometer inserted into meat loaf reads 160°, 24-28 minutes., Meanwhile, warm remaining sauce over low heat; stir in parsley. Serve sauce with Wellingtons.