Heat oven to 400 degrees F. Cook and stir chicken in dressing in large skillet on medium heat 5 min. or until done. Stir in vegetables, soup and VELVEETA. Spoon into greased 9-inch square baking dish.
Unfold pastry sheet; place over chicken mixture. Fold under edges of pastry; press onto top of baking dish to seal. Brush with egg. Cut several slits in crust to permit steam to escape. Place on baking sheet.
Bake 30 min. or until crust is deep golden brown. Let stand 5 min.