Heat oven to 450°F. Remove crust from pouch; place on ungreased large cookie sheet. Place roast beef, tomato, onion and cheese lengthwise down center 1/3 of crust to within 1 inch of top and bottom edge.
Brush edge of crust with water. Fold top and bottom edges over filling; fold in sides, overlapping in center and sealing edges. Cut 3 or 4 slits in top crust to allow steam to escape.
Bake 17 to 20 minutes or until golden brown. Cool 5 minutes. Cut crosswise into 8 slices. Serve warm.