Melt two tablespoons 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.
Saute 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 degrees 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 broth, 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, as well as 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. I recommend placing the dish on a sheet pan to make sure nothing drips and makes a mess in the oven. Then, serve and enjoy!
Notes
*You need two single pie crusts, or one double pie crust. Storing leftovers: Keep in an airtight container in the fridge for up to 7 days.Freezing: You can either freeze leftovers for up to 3 months, or you can freeze an unbaked pie in the actual pie plate ready to go, then bake for an extra 15 minutes. To thaw fully cooked leftovers, leave in the fridge overnight. Or, you may reheat cooked leftovers straight from frozen by adding 5 minutes of extra oven time, or 30-60 seconds of extra microwave time.Reheating: The microwave is the most convenient, though the crust will not be as crisp as the oven method. Microwave in 30 second intervals until warmed through. Alternatively, you can heat a serving in a 300F oven for 15 minutes, or until warmed through. If reheating the whole dish, it will probably take closer to 25 minutes. Make-ahead option: There are a few options for doing this. The two I recommend are to 1) make and chill the filling as well as the crusts, assemble it all with cold ingredients, and pop the entire unbaked pie into the freezer, then bake when you’re ready. You will likely need an extra 15-20 minutes of bake time, if baking straight from frozen. The other option 2) is to make the filling and refrigerate it, and prepare and refrigerate the crusts ahead of time as well. Then simply assemble everything in the pie plate the next day, and bake.