Heat oven to 425°F. Prepare pie crusts as directed on package for two-crust pie using 9-inch glass pie pan.
In large nonstick skillet, cook beef and onions over medium-high heat for 4 to 6 minutes or until beef is browned, stirring frequently. Stir in potatoes, carrot, peas and mushrooms.
In small bowl, combine gravy, cornstarch, thyme, salt and pepper; mix well. Stir into beef mixture; cook until thoroughly heated. Pour mixture into crust-lined pan. Top with second crust; seal edges and flute. Cut small slits in several places in top crust.
In another small bowl, combine egg yolk and water; blend well. Brush top crust with egg mixture; sprinkle with sesame seed.
Bake at 425°F. for 30 to 40 minutes or until crust is golden brown and filling is bubbly. Cover edge of crust with strips of foil after first 15 to 20 minutes of baking to prevent excessive browning. Cool 10 minutes before serving.