Grease the bottom of a 15 1/2 x 10 1/2-inch jellyroll pan with butter.
It is easiest to make this pastry in a food processor. In a small bowl, mix egg and sour cream. In the food processor, pulse the flour and salt until mixed. Add butter; process 15 seconds, until it has the consistency of coarse meal.
With processor running, add egg mixture; process just until dough comes together. If you don't have a food processor, chill a large bowl, mix the flour and salt in the bowl, then cut in butter with a pastry blender or two forks. Stir in egg mixture just until you can form the dough into a ball. Wrap dough in waxed paper; refrigerate for 1 hour.
Divide dough in half. Roll each half into a 14 x 6-inch rectangle. Place one rectangle in prepared pan; set pan and remaining pastry aside.
Heat oven to 375°F Melt butter in a large skillet. Add mushrooms; cook over medium heat for 6 minutes or until slightly colored.
Add ground meat; cook until browned and until liquid in pan cooks away. Place meat mixture in a large bowl. Add onions, parsley, cheese and milk; mix well.
Gather mixture into a ball; place onto dough rectangle in pan. Using your hands, press meat into a narrow loaf shape. Brush edges of dough with egg-milk mixture. Place the second dough rectangle over the meat. Using a fork, press the dough edges together to seal. Brush top of pastry with egg-milk mixture. Prick top with a fork in several places. Bake 45 minutes, until golden brown.