Sprinkle steak strips with half the pepper. Melt 1 T butter in large, preferably nonstick, skillet over med-high heat. Add sliced mushrooms, whites of scallions and remaining pepper. Cook, stirring often, until mushrooms are tender, about 8 minutes. Remove with slotted spoon to bowl. Wipe skillet clean. Add remaining butter and heat over high heat until sizzling. Add beef strips and stir-fry about 2 minutes until browned. Stir in gravy and vinegar; bring to a boil. Remove from heat and stir in sour cream until blended. Sprinkle with green part of scallions. Serve immediately. Per serving: 184 cal, 19 g. pro, 6 g carb, 10 g fat, 342 mg sodium. Reprinted from Woman's Day magazine, April 1996.