Preheat the oven to 400°F (200°C).
Melt the butter in a large skillet over medium heat.
Add the diced onion and cook for 3–4 minutes, until softened.
Sprinkle the flour over the onions and butter. Stir continuously for about 1 minute.
Gradually add the chicken broth while stirring until smooth.
Pour in the milk and continue stirring until the sauce thickens.
Add the cooked chicken and frozen mixed vegetables.
Stir in the garlic powder, thyme, salt, and black pepper. Cook for another 3–5 minutes until everything is heated through.
Transfer the chicken mixture to a pie dish and spread it evenly.
Place the pie crust over the filling and gently press the edges around the dish.
Cut a few small vents in the top of the crust.
Brush the crust lightly with the beaten egg.
Bake for 30–35 minutes, or until the crust is golden brown and the filling is bubbling.
Remove from the oven and let the pot pie rest for 5–10 minutes before serving.
