Making Creamy Chicken Pot Pie can be quite simple when you follow these steps:

1. Preheat Oven: Start by preheating your oven to 425°F (220°C).
2. Cook Vegetables: In a medium saucepan, melt the butter over medium heat. Add the onion, celery, peas, and carrots. Cook until the onions are translucent.
3. Make the Roux: Stir in the flour, garlic powder, thyme, salt, and pepper. Cook for about 1-2 minutes, stirring constantly.
4. Add Liquids: Gradually pour in the chicken broth and milk. Stir continuously until the mixture thickens, about 3-5 minutes.
5. Combine Ingredients: Remove from heat and add the cooked chicken and diced potatoes (if using). Mix well to combine.
6. Prepare Pie Crust: Place one pie crust into a 9-inch pie pan, pressing gently into the sides. Pour the chicken mixture into the crust.
7. Top with Second Crust: Place the second pie crust over the filling. Seal the edges by crimping with a fork or pinching with your fingers. Cut a few slits in the top crust to allow steam to escape.
8. Apply Egg Wash: Brush the top crust with the beaten egg for a beautiful golden crust.
9. Bake: Place the pie in the preheated oven and bake for 30-40 minutes, or until the crust is golden brown and the filling is bubbly.
10. Cool Slightly: Once baked, remove the pie from the oven and let it cool for about 10 minutes before serving to allow the filling to set.

These clear steps will guide you in creating a delightful Creamy Chicken Pot Pie that will impress your family and friends.