Preheat oven to 425°F. Place one pie crust in 9-inch pie pan.
In medium saucepan, melt butter over medium heat. Add onion; cook until tender, about 2 minutes. Stir in flour, salt and pepper. Cook stirring constantly, until mixture is bubbly, remove from heat. Stir in broth and milk. Heat to boiling, stirring constantly. Boil and stir for 1 minute. Stir in tuna and vegetables; remove from heat.
Spoon tuna filling over crust. Place second crust over filling; flute edge. Cut slits in several places.
Bake 425°F for 35 to 40 minutes or until crust is golden brown.