Part 1: Make the All-Butter Pie Crust

(This can be done up to 2 days ahead and refrigerated, or frozen for longer)

1. Combine Dry Ingredients: In a large bowl, whisk together the 2 ½ cups of flour and 1 teaspoon of salt.
2. Cut in the Butter: Add the cold butter cubes to the flour mixture. Using a pastry blender, two knives, or your fingertips, quickly cut the butter into the flour until the mixture resembles coarse meal with some pea-sized pieces of butter still visible. Work quickly to keep the butter cold. Large butter pieces = flaky layers.
3. Add Ice Water: Sprinkle about ½ cup (120ml) of ice water over the mixture. Using a fork or your hands, gently mix until the dough just starts to come together. Don’t add all the water at once. If it seems too dry, add more ice water, 1 tablespoon at a time, mixing just until the dough holds together when squeezed. Do NOT overmix or knead the dough – this develops gluten and makes the crust tough.
4. Divide and Chill: Turn the shaggy dough out onto a lightly floured surface. Gently bring it together into a cohesive mass (don’t knead!). Divide the dough in half and flatten each half into a 1-inch thick disk. Wrap each disk tightly in plastic wrap.
5. Refrigerate: Chill the dough disks in the refrigerator for at least 1 hour (or up to 2 days) before rolling. This allows the gluten to relax and the fat to solidify, which is crucial for flakiness and makes the dough easier to handle.

Part 2: Prepare the Creamy Chicken Filling

(This can also be made a day ahead and refrigerated. Reheat gently before assembling or let come closer to room temp.)

1. Sauté Aromatics: Melt the 6 tablespoons of butter in a large pot or Dutch oven over medium heat. Add the chopped onion, diced carrots, and diced celery. Cook, stirring occasionally, until the vegetables soften, about 8-10 minutes. (If adding mushrooms or potatoes, add them here and cook until tender/softened).
2. Make the Roux: Sprinkle the ½ cup of flour over the cooked vegetables. Stir constantly and cook for 1-2 minutes. This cooks out the raw flour taste and forms a roux, which will thicken the sauce.
3. Add Liquids Gradually: Slowly whisk in the warm chicken broth, a little at a time, ensuring there are no lumps. Once all the broth is incorporated and smooth, slowly whisk in the warm heavy cream or milk.
4. Simmer and Thicken: Bring the mixture to a gentle simmer, stirring frequently. Reduce the heat to low and continue simmering, stirring often (especially the bottom to prevent scorching), until the sauce has thickened enough to coat the back of a spoon, about 5-8 minutes.
5. Season and Add Fillings: Stir in the salt, black pepper, and dried thyme. Taste and adjust seasonings if needed – remember the chicken will add some savoriness, but the crust is plain, so the filling needs to be well-seasoned.
6. Fold in Chicken and Peas: Remove the pot from the heat. Gently fold in the cooked chicken and frozen peas. Stir in the fresh parsley, if using.
7. Cool Slightly: Set the filling aside to cool for at least 15-20 minutes (or longer if possible). Putting hot filling onto raw pastry can melt the butter and lead to a soggy bottom crust. Ideally, the filling should be lukewarm or room temperature.

Part 3: Assemble and Bake the Pot Pie

1. Preheat Oven & Prepare Dish: Preheat your oven to 400°F (200°C). Have a standard 9-inch pie dish ready (glass, ceramic, or metal). You don’t typically need to grease it for a butter crust.
2. Roll Bottom Crust: On a lightly floured surface, take one chilled dough disk. Roll it out from the center outwards into a circle about 12-13 inches in diameter and roughly ⅛-inch thick. Rotate the dough occasionally and add minimal flour as needed to prevent sticking.
3. Line the Pie Dish: Carefully transfer the rolled-out dough to the pie dish. Gently press it into the bottom and up the sides, ensuring there are no air pockets. Trim the excess dough hanging over the edge, leaving about a ½-inch overhang.
4. Add Filling: Pour the slightly cooled chicken filling into the pastry-lined dish, spreading it evenly.
5. Roll Top Crust: Roll out the second disk of dough similarly to the first, large enough to cover the top of the pie (about 11-12 inches).
6. Place Top Crust: Gently lay the top crust over the filling.
7. Trim and Seal: Trim the top crust edge so it matches the overhang of the bottom crust (about ½ inch). Fold the edge of the top crust under the edge of the bottom crust. Press the edges together firmly to seal, then crimp decoratively using your fingers or the tines of a fork.
8. Cut Vents: Using a sharp knife, cut several slits (at least 4-5) in the top crust. This allows steam to escape during baking, preventing the filling from bursting through the sides.
9. Optional Egg Wash: For a shiny, deep golden crust, brush the top crust evenly with the prepared egg wash. Avoid letting it pool too much around the edges.
10. Bake: Place the pie on a baking sheet (to catch any potential drips). Bake in the preheated 400°F (200°C) oven for 20 minutes.
11. Reduce Heat & Continue Baking: After 20 minutes, reduce the oven temperature to 375°F (190°C). Continue baking for another 30-40 minutes, or until the crust is deeply golden brown and the filling is bubbling gently through the vents. If the crust edges start browning too quickly, you can shield them with strips of aluminum foil or a pie crust shield.
12. Rest Before Serving: This is important! Once baked, carefully remove the pie from the oven and let it rest on a wire rack for at least 15-20 minutes (ideally 30 minutes) before slicing and serving. This allows the creamy filling to set slightly, making it easier to slice neatly.