Your family will love this new spin on calzones, and you'll love the ease and convenience.
Preheat oven to 400°. Unroll 1 piecrust on a parchment paper-lined baking sheet. Spread 1 cup potatoes over half of crust, leaving a 1-inch border around edge. Toss together chopped roast and next 3 ingredients; spoon half of mixture over potatoes. Whisk together egg and 1 tsp. water; brush a small amount of egg mixture around edge of piecrust. Fold dough over filling, pressing and folding edges to seal. Repeat with remaining crust, potatoes, pot roast mixture, and a small amount of egg mixture. Brush tops of pies with remaining egg mixture; cut small slits in top for steam to escape.
Bake at 400° for 25 to 30 minutes or until crusts are golden brown and crisp.