Heat oven to 425°F. Make pie crusts as directed on box for Two-Crust Pie using 9-inch glass pie pan.
In 3-quart saucepan, mix gravy and cornstarch until well blended. Stir in Worcestershire sauce, frozen vegetables and mushrooms. Cook over medium-high heat, stirring occasionally, until bubbly. Stir in roast beef. Pour into crust-lined pan.
Top with second pie crust; seal edge and flute. Cut decorative slits in several places in top crust to allow steam to escape.
Bake 15 minutes. Cover crust edge with strips of foil to prevent excessive browning; bake 15 to 30 minutes longer or until crust is golden brown and filling is bubbly.