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