Melt butter in a dutch oven and cook onions for 3 minutes until softened. Add garlic and cook for 2 minutes.
Stir in flour and constantly whisk for 2 minutes to cook. Slowly add in chicken broth stirring constantly to get out any lumps. Gravy will thicken.
Add the milk and mix together. Add in seasonings and adjust if needed.
Mix in frozen peas and carrots and cook for 2 minutes. Then add in chicken and combine. Turn off heat.
Place one sheet of pie crust in the dish. Fill with the pot pie filling. Place other sheet of pie crust over the top of filling. Seal edges with a fork and put slits in the center so it can vent. Brush with egg wash.
Place pie in a preheated 400° oven for 30-35 minutes.
Let sit for 10-15 minutes. Slice and enjoy!