Open foil bag. Arrange celery, red pepper and onion in foil in even layer. Combine sauce mix, flour and water. Pour mixture over vegetables in foil bag. Sprinkle each piece of steak with seasoned salt, then press pepper on top side of each piece.
Place steaks on vegetables in even layer.
To seal, double-fold open end of foil bag. Place foil bag in 1-inch deep pan. To cook, slide foil bag onto grill or leave foil bag in supporting pan and place in oven. Use heavy oven mitts to cut open foil with sharp knife. Carefully fold back top of foil bag, allowing steam to escape.