01 - Preheat your oven to 400°F.
02 - In a large skillet over medium heat, melt 4 tablespoons of unsalted butter. Add the diced onions, carrots, celery, and potatoes (if using). Sauté for 5–7 minutes until the vegetables begin to soften.
03 - Stir in the minced garlic and continue to cook for 1 minute until fragrant.
04 - Sprinkle 1/3 cup of all-purpose flour over the sautéed vegetables and stir thoroughly to coat. Cook for 1–2 minutes, allowing the flour to lightly toast.
05 - Gradually whisk in 2 cups of chicken broth and 1 cup of whole milk. Continue stirring constantly until the mixture thickens to a rich, creamy sauce, which typically takes about 3–5 minutes.
06 - Stir in 1/2 teaspoon of salt, 1/4 teaspoon of black pepper, 1/2 teaspoon of dried thyme, and 1/2 teaspoon of dried parsley. Fold in the cooked, diced chicken and frozen peas. Allow the mixture to simmer for 2–3 minutes, then remove the skillet from the heat.
07 - Pour the prepared chicken and vegetable filling evenly into a 9-inch pie dish or baking dish.
08 - Carefully unroll or roll out the ready-made pastry sheet to fit precisely over the filled dish. Place the pastry over the filling, gently pressing the edges to seal it securely to the rim of the dish. Trim away any excess pastry for a clean finish.
09 - Using a sharp knife, make a few small slits in the top of the pastry crust to allow steam to escape during baking. Brush the entire surface of the pastry with the beaten egg for a beautiful golden sheen.
10 - Bake in the preheated oven for 30–35 minutes, or until the pastry crust is puffed, golden brown, and the filling is visibly bubbling.
11 - Allow the chicken pot pie to rest for 10 minutes after baking before slicing and serving. This crucial step helps the filling set and prevents it from being overly runny.