Set your oven to 425°F (220°C).
Melt butter in a pan over medium heat. Add onion and celery, cook 3–4 minutes until soft. Stir in flour and cook 1 minute.
Slowly whisk in broth and milk. Cook 3–5 minutes until thick. Add chicken, veggies, garlic powder, thyme, salt, and pepper. Mix well and take off the heat.
Roll out pie crust and cut 12 circles (4–5 inches wide). Press them into muffin tin cups.
Spoon the chicken mix into each crust, almost to the top.
Cut 12 more dough circles or shapes. Place on top, seal the edges with your fingers or a fork. Brush with beaten egg if you like.
Put the muffin tin on a baking sheet. Bake for 15–20 minutes until golden and bubbly.
Let cool 5 minutes. Serve warm as-is or with a salad.