Melt two Tbsp. of butter over medium heat in a large pot and add the onion, carrot, celery, potato, sage, thyme, rosemary, 1/4 tsp. salt, and 1/8 tsp. black pepper.
Sauté the vegetables, stirring every few minutes until soft, about 20 minutes. Add the peas and diced chicken to the pot.
Preheat the oven to 375˚F.
Add the remaining 6 Tbsp. of butter to the softened vegetables and let it melt completely. Add the flour and stir to distribute until there are no more pockets of dry flour. Remove the herb stems from the mixture and discard.
Add the chicken stock, bring the mixture to a boil, reduce the heat to a simmer, and let it bubble for a minute or two until the mixture has thickened. Stir in the heavy cream, Vadouvan, 1/2 tsp. salt and 1/4 tsp. black pepper (or desired amounts to taste). Remove the pan from the heat.
Lay the bottom pie crust into a standard 9" pie plate and spoon all of the chicken vegetable mixture into the crust. Place the other pie crust on top, and crimp with your fingers or with the tines of a fork. Brush the top with egg wash, then cut slits into the top of the crust so the steam can escape during baking.
Bake for one hour, until the pie crust is deeply golden brown and flaky. Serve and enjoy!