In a large skillet melt butter over medium heat. Add onion, celery, and carrot and cook 4 to 5 minutes.
Sprinkle flour on top of onion mixture. Cook and stir for 1 minute.
Gradually whisk in chicken broth and half-and-half.
Add cream cheese, breaking it apart with a wooden spoon so that it melts and blends into the sauce. Cook until bubbling and thickened, about 1 to 2 minutes.
Add salt, garlic powder, thyme, and pepper.
Stir in chicken and peas. Remove from heat.
Preheat oven to 400 degrees and spray a baking sheet with cooking spray.
Unroll piecrusts and use a rolling pin to roll them slightly thinner. You want to be able to cut 4 circles out of each pie crust.
Use a bowl that's approximately 5 to 6 inches across to cut a total of 8 circles. Roll the scraps together and cut 2 more circles.
Place filling(as much as you can and still be able to fold over the dough and seal it) on one side of a circle of dough.
In a small bowl, whisk together egg white and milk. Use your finger or a small pastry brush to wet the edges of the dough with egg wash so that they will stick together. Fold dough over, pressing to seal edges. Crimp around edges with a fork.
Repeat with remaining dough circles. Place on prepared baking sheet and brush tops with egg wash.
Place in oven and bake for 15 to 17 minutes.