While dough chills, in a large bowl, combine beef with potato, turnip, carrot and onion, and season generously with salt and pepper, thyme and rosemary.
Pre-heat oven to 400°F and line a baking sheet with parchment paper.
Remove dough from refrigerator and cut it into 6 equal pieces.
On a floured surface, roll each piece out to an 8-inch circle.
Spoon 1/2 - 2/3 cup beef mixture into the center of dough, then top with 1/2 tablespoon butter.
Take the far edge of your dough circle and fold it over the beef, towards yourself, to form a half moon.
Seal edges with a fork and repeat with remaining pasties.
Transfer to lined baking sheet and brush the tops of dough with beaten egg.
Bake for 20 minutes, or until lightly browned, then lower oven temperature to 350º F and bake for another 15 - 20 minutes, or until golden brown.
Serve hot with brown mustard, optional.