Creating a Chicken Potpie can be simple if you follow these steps:

1. Preheat the Oven: Begin by preheating your oven to 425°F (220°C).
2. Cook the Vegetables: In a large skillet, melt the butter over medium heat. Add the onions, carrots, and celery, cooking until softened, about 5-7 minutes.
3. Make the Sauce: Stir in the flour and cook for 1 minute. Gradually whisk in the chicken broth and milk. Cook until the mixture thickens, about 3-5 minutes.
4. Season the Filling: Add the diced chicken, peas, salt, pepper, garlic powder, and thyme. Mix well, then remove from heat.
5. Prepare the Pie Crust: Roll out one pie crust and fit it into a 9-inch pie dish. Pour the chicken filling into the crust.
6. Top with Second Crust: Roll out the second pie crust and place it over the filling. Seal the edges by crimping with a fork or your fingers. Cut slits in the top crust to allow steam to escape.
7. Brush with Egg Wash: Brush the top crust with the beaten egg for a golden finish.
8. Bake: Place the pie in the preheated oven and bake for 40-50 minutes, or until the crust is golden brown and the filling is bubbly.
9. Cool: Allow the potpie to cool for about 10 minutes before serving, as the filling will be very hot.

These steps will guide you in creating an amazing Chicken Potpie that will surely impress your family and friends.