1. Make the filling - in a large saucepan over medium-high heat melt 2 tablespoons of butter. Stir in the onion, celery, carrot, potato, salt and Herbes de Provence seasoning. Saute for 15 minutes or until vegetables are tender.
2. Stir in the peas and chicken.
3. Add the remaining 4 tablespoons butter to the pan. Once melted sprinkle the flour over the chicken mixture and stir to combine. Cook for 2 minutes.
4. Pour in the stock and bring to a boil. Then simmer for 5 minutes or until the filling has thickened.
5. Allow the filling to cool completely, then refrigerate for 2 hours or overnight.
6. Preheat oven to 400º degrees F. Line a rimmed baking sheet with a silicone mat or parchment paper.
7. Roll out the pie crusts to 1/8 inch thick and cut out eight 6 inch circles. ( I always trace a plate). Reroll scraps as needed but not more than once.
8. Add 3-4 tablespoons of the filling to the middle of each circle, fold the dough in half and crimp the edges to seal with your fingertips or use the tines of a fork. At this point the pies can be frozen. Thaw overnight in the refrigerator, then follow the remaining instructions to bake.
9. Transfer to the prepared pan. Cut a slit in the middle of each pie to allow steam to vent. Brush with egg wash. Repeat with remaining pie dough and filling.
10. Bake for 25-30 minutes or until the crust is golden brown. Cool for 5-10 minutes before serving.