Heat oven to 425°F.
In medium saucepan, melt butter over medium heat. Add onion; cook 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; mix well. Remove from heat. Spoon into crust-lined pie pan. Top with second crust; seal edges and flute. Cut slits or shapes in several places in top of crust.
Bake 30 to 40 minutes or until crust is golden brown. Let stand 5 minutes before serving.