Sift the flour and salt together into a large bowl. With 2 knives cut in the butter until the mixture is like coarse meal.
In a separate bowl mix the egg and sour cream. Add this to the flour mixture until the dough forms a soft, pliable ball. Wrap the dough in wax paper and refrigerate for 1 hour.
After chilling, divide the dough in half and roll out each half to a rectangle 6"x14". Save any scraps of dough. Butter the bottom of a flat cookie sheet or jelly-roll pan. Put one sheet of the pastry onto the pan.
Melt butter in a 12" skillet. Add the mushrooms and cook them for 6 - 8 minutes, until lightly colored. Add the meat and continue to cook until the meat loses all trace of pink color. Combine the mushrooms, cooked meat, onion, parsley, cheese, and milk in a large mixing bowl.
Form the meat mixture into a ball and put it in the center of the dough. With your hands, very gently pat the meat into a loaf shape. Cover this with the second piece of pastry dough and seal the edges with a fork.
Moisten the dough with a pastry brush dipped in the egg and 2 Tbl milk which has been beaten together. Prick holes in the top of the loaf to allow steam to escape.
Roll out the scraps of dough and cut them into strips. Crisscross these over the top of the loaf to form a design. Again brush it with the egg/milk mixture.
Bake the loaf at 375°F for 45 minutes, when the pastry will be golden brown.