Heat oven to 425. Prepare pie crusts as directed on package for two-crust pie using 9 inch pie pan.
Melt butter in a medium saucepan over medium heat. Add onion and cook for 2 minutes or until tender. Add flour, salt and pepper; stir until well blended. Gradually stir in broth and milk, cooking and stirring until bubbly and thickened.
Add chicken and mixed vegetables and mix well. Remove from heat. Spoon mixture into crust-lined pan. top with second crust; seal edges and flute. Cut slits in several places in top crust.
Bake at 425 for 30 to 40 minutes or until crust is golden brown. Let stand 5 minutes before serving.