Preheat oven to 425
Combine chicken, carrots, peas, and celery in a saucepan and add water just until the mixture is covered.
Bring to boil and cook for 15 minutes, drain, set aside. If using rotisserie chicken skip this step
Bring to boil and cook for 15 minutes, drain, set aside.
In a separate pan, cook onions in butter until translucent
Add flour and seasonings. Slowly stir in chicken broth and milk (making a roux).
Simmer on med-low heat until thick then remove from heat.
Add cooked veggies and chicken (or rotisserie chicken) to mixture.
Pour mixture in pie crust and top with the second crust. Seal edges well. Mix one egg yolk with 1 Tbs water to wash the crust.
Bake until golden brown, about 25-30 minutes. Let cool 10 minutes