In a bowl or in a food processor, mix flour and salt together before adding in cold, cubed butter. Pulse for 10-15 seconds, or until mixture is crumbly and butter is the size of small pebbles. With a motor running, slowly drizzle in 2/3 cups ice water and pulse until a dough ball begins to form. Once the dough is formed, turn it out onto a lightly flowered surface and roll it into a large rectangle. Wrap dough tightly in plastic wrap and refrigerate 30-45 minutes. or until chilled. While dough chills, combine beef with potato, carrot and onion, and season generously with salt and pepper, thyme and rosemary. Preheat oven to 400 degrees and line a baking sheet with parchment paper. remove dough from refrigerator and cut it into 6 equal parts. on a floured surface roll each piece out into an 8- inch circle. Spoon 1/2-2/3 cup beef mixture into the center of the dough. then top with 1/ tablespoons of 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 repeats for remaining pieces. Transfer to baking sheet and bake for 20 minutes or until lightly browned. you may also add a small piece of fat to the pastery before baking. Serve hot with brown mustard and enjoy.