Make or thaw pie crust.
Preheat oven to 400 degrees.
Saute onion in butter until soft.
Add flour and cook for 2-3 minutes.
Add broth, milk, and seasoning. Whisk until smooth.
Add in chicken and vegetables, cook over low heat until thick and bubbling.
Pour filling into pie pan or casserole dish and top with pie crust.
Brush beaten egg onto pie crust.
Bake for 30 minutes until crust is golden brown and filling is bubbling.