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