While pulsing, slowly pour in 1/2 cup of the buttermilk and continue to pulse until the buttermilk is incorporated into the dough. At this point, your dough should look like rough crumbs and hold their shape if squeezed together. If dough does not hold its shape, drizzle in a little more buttermilk, up to 2/3 cup until it does. The dough will look a little scary at this point, but don't worry. It's supposed to look like that!
Lay a sheet of plastic wrap on the counter and dump half of your dough crumbs onto the sheet. Gather the edges and press tightly together to form a 1-inch thick disk. Do the same with the rest of the dough. Place in a plastic bag and refrigerate for at least an hour, up to 24.
In a medium saucepan, melt butter over medium-high heat (watch closely, you don't want to burn the butter) and add the meat. Cook until well browned.
Add potatoes and carrots and cover with just enough water that the meat and vegetables are submerged. Add salt, rosemary, pepper, and oregano.
Remove from heat and drain the cooking liquid into a separate container for later use. Set liquid and meat and veggies aside.
In a medium saucepan, melt remaining 3 tablespoons of butter over medium. Add onions and cook until they begin to soften, about 2 to 3 minutes. Add flour and whisk continuously for 1 minute until a sticky paste forms and flour turns golden. Whisk in 1 cup of the reserved broth (discard the rest). Simmer for 2 to 4 minutes, whisking continuously, until a thick gravy forms. Remove from heat and stir in the meat and vegetable mixture, making sure everything is well coated.
Whisk together egg and milk in a small bowl to make your egg wash.
On a well-floured surface, divide your dough into 4 to 6 equal-size pieces and roll into balls. If you are slow at rolling out pastry dough, place all the balls, but one, back in the fridge and work with one at a time so the dough stays chilled and easy to work with.
Brush egg wash around the edges of your pie crust to help seal it and spoon beef mixture over slightly less than half of the pie, leaving a 1/2-inch border. Fold pie over and use a fork to crimp the edges closed. Cut 3 slashes in the top and transfer to baking pan.
Brush pies with egg wash and and bake for 15 to 20 minutes, until pies are beautiful and golden.
*If you are sensitive to salt, cut salt in the filling to 1 teaspoon. These are fairly salty pasties.