To make the chicken pot pie filling, melt 2 tablespoons butter over medium heat in a large pot. Add the chopped celery, carrot, potato, onion, dried thyme, dried rosemary, and salt. Saute until soft, about 15 minutes. Add the peas and diced chicken to the pot.Add the remaining 4 tablespoons butter to the softened vegetables, and let it melt completely. Add the flour and stir to distribute, then add the chicken stock. Bring the mixture to a boil, then back down to a simmer, and let it bubble for five minutes until it has thickened. Adjust the seasoning if desired, then let this mixture cool to room temperature, then chill completely in the fridge, for at least 3 hours.In the meantime, make the pie crust. Pulse the flour and salt together in a food processor 10 times, to combine. Add the cubed butter and pulse about 20 times until the butter is chopped up into pea-sized pieces. Drizzle in the ice water and pulse until the dough comes together. You want to stop adding water when it looks crumbly, but if you pinch a bit of the dough, it holds together. Dump the dough out onto the counter, bring it together into a flat disk with your fingers, then wrap in plastic. Chill in the fridge for two hours.Once the filling and pie crust are chilled, we’re ready to bake. Preheat the oven to 400ºF.Roll the pie crust out to about 1/8 inch thick, and cut 6-inch wide circles. If you re-roll the scraps, you should be able to get 8 circles total.Place a few spoonfuls of filling into each circle, then enclose by crimping with your fingers. Repeat with the remaining ingredients, then place four pies each on two sheet pans. Cut vent slits into each pie, then brush with egg wash. Bake for 25–30 minutes until golden brown and crusty. Let the pies cool for at least 5 minutes, then enjoy! Note: You can use your own pie crust recipe or store-bought pie crust, if desired.