In a large, deep skillet, melt 4 tablespoons of butter over medium heat.Add the onion and sauté, stirring frequently, for 5-6 minutes, or until softened. Add the celery and carrots, and stir to combine. Add the thyme, season generously with salt and pepper, and sauté for 6-7 minutes, stirring occasionally.
Step 2
Add the remaining butter, allowing it to melt before adding the flour. Stir to coat all ingredients, and allow to cook until ingredients have begun to brown, around 5 minutes.Slowly stir in the warmed milk and bring to a simmer. Add the chicken base and continue to stir; the sauce will thicken.
Step 3
Add the potatoes and chicken, and simmer for 2 minutes. Stir in the peas, and turn off the heat. Taste and correct seasoning, and set aside to cool, then transfer to the refrigerator. Do not fill the crust until the filling is cold.
Step 4
Next, make your pie crust. Set a box grater over a large, shallow dish. Shred the cold butter as if you were shredding cheese. Cover the dish with plastic wrap and set it in the freezer or refrigerator to stay cold.
Step 5
Mix the dry ingredients together in a very large bowl and set aside.
Using a silicone spatula, stir in the cold butter, distributing it as evenly as you can. Pour the water over the dough and mix to combine until it is the consistency of dough.
Step 6
Form the dough into two discs. Wrap each with plastic wrap and transfer to the refrigerator to chill for 20 minutes.
Step 7
Move one of the rounds of dough to a well-floured countertop, leaving the remaining dough in the refrigerator.
Roll out to a bit larger than the size of your pie pan, and transfer to the pie pan. Fill your crust with the chilled filling.
Now roll out the remaining dough and lay it atop your filling as the top crust. Crimp the edges and cut a few slits on the top crust for venting.Transfer the pie to the freezer to chill for 30 minutes. While the pie is chilling, preheat your oven to 425°F (220°C).
Step 8
Place the pie on a large sheet pan lined with foil for easy cleanup.Place the pie in the oven and bake for 20 minutes, then reduce the heat to 375°F (190°C) and continue to bake for 45-55 minutes or until nicely browned. If the crust is browning faster than desired, you may tent it loosely with foil.