Preheat the oven to 375°F (190°C).

In a large skillet, melt butter over medium heat. Add the diced onion, celery, and frozen peas and carrots. Cook for 5-7 minutes until vegetables are softened.

Sprinkle the flour over the veggies and stir to combine. Cook for another 1-2 minutes to form a roux.

Gradually add the chicken broth while stirring, followed by the heavy cream. Stir until the sauce thickens (about 3-5 minutes).

Stir in the cooked chicken, garlic powder, thyme, salt, and pepper. Remove from heat and set aside.

Roll out the pie crust and cut out circles large enough to fit your muffin tin or mini pie dishes. Press the dough into the tin, ensuring it fits the sides.

Spoon the chicken mixture into the crusts, filling them evenly.

Cut smaller rounds of pie crust to top the pies. Place them over the filling and crimp the edges to seal.

Brush the top with a beaten egg for a golden finish.

Bake in the preheated oven for 25-30 minutes, or until the crust is golden brown.

Allow the pies to cool for 5 minutes before serving.