Dough
Blend together flour, salt and butter with pastry blender or two knives used scissors fashion till mixture resembles coarse meal. Beat together yolk and 1/3 cup ice water. Drizzle over flour mixture and gently stir with fork till incorporated. Gently squeeze small handful of dough. If it doesn't hold together, add more ice water, 1/2 tablespoon at a time till it does. If dough is overworked or has too much water it will be tough. Turn out onto lightly floured surface and divide into 6 portions. With floured heel of hand, smear each portion once in a forward motion to help distribute butter. Form each portion into disk, wrap each in plastic wrap and chill 1-8 hours till firm.
Filling
Heat 1 tablespoon oil in skillet over medium-high heat till hot but not smoking. Saute mushrooms 6 minutes, stirring till browned and dry. Transfer to large bowl. Add remaining oil to skillet and cook onion and garlic over medium heat 5 minutes, stirring, till softened. Add onion to mushrooms. Peel potato and cut into 1/2 inch cubes and add to mushroom mixture. Add steak, carrots, parsley, salt and pepper, tossing well. Cool completely.
Assembly
Butter baking sheets. Roll out one piece of dough on lightly floured surface to 9 inch round. Mound 1 cup of filling on dough round, slightly off center. Brush a 1/2 inch border on dough with water and fold dough over to form half circle. Trim edge even with fluted pastry wheel or knife and crimp to seal. Cut 3 small steam vents in top crust with knife and put pasty on prepared sheets. Repeat with remaining dough and filling. Chill 30 minutes, loosely covered. Heat oven to 350°F. Brush pasties with beaten egg and bake 40 minutes till golden. Cool slightly before serving.