Preheat the Oven: Preheat oven to 400°F. Lightly grease a 13 x 18-inch sheet pan with a 1-inch rim.
Cook the Vegetables: In a large skillet, melt the 4 tablespoons unsalted butter over medium heat. Add the 3 large carrots, 2 large stalks celery, 1 large yellow onion, and 3 cloves garlic. Sauté until softened and tender-crisp, about 5–8 minutes.
Make the Sauce: Sprinkle 1/3 cup all-purpose flour, 2 teaspoons kosher salt, 1/2 teaspoon black pepper, 1/2 teaspoon dried thyme, and 1/2 teaspoon dried parsley over the vegetables. Stir and cook for 2 minutes. Slowly stir in the 2 cups chicken stock and 1 cup heavy cream. Cook, stirring frequently, until thick and bubbly, about 5–6 minutes.
Add Chicken and Peas: Stir in the 4 cups cooked chicken and 1 cup frozen peas. Taste and adjust seasonings as needed. Remove from heat and allow the filling to cool slightly while preparing the crust.
Prepare the Crust: Roll out 2 refrigerated pie crusts into a 10 x 15-inch rectangle. Using a pizza cutter or knife, cut into 1-inch strips (or use cookie cutters for fun shapes).
Assemble the Pot Pie: Spread the chicken mixture evenly in the sheet pan. Lay crust strips across the short side of the pan, leaving gaps for steam to escape. Press strips lightly to the pan edges. Brush with egg wash.
Bake: Place a foil-lined baking sheet on the rack beneath the pie to catch spills. Bake uncovered for 30–35 minutes, or until crust is golden brown and filling is bubbling.
Rest and Serve: Let cool 10–15 minutes before slicing. Garnish with fresh parsley if desired.
Notes
Make-Ahead Tips: The filling can be prepared up to 2 days in advance. Store covered in the refrigerator, then assemble with crust when ready to bake.Freezing Tips: Assemble but don’t add egg wash. Cover tightly with foil and freeze for up to 3 months. Thaw overnight in the fridge, brush with egg wash, and bake as directed.Storage: Leftovers keep refrigerated in an airtight container for up to 4 days.Reheating: Microwave single portions for 1–2 minutes or reheat in the oven at 350°F until warmed through.Customization: Swap the chicken for leftover turkey, or change up the vegetables with corn, mushrooms, or green beans. Puff pastry also makes a fabulous crust alternative.