Rub steak on both sides with steak sauce; let stand for 10 minutes., Meanwhile, in a small skillet, saute mushrooms in butter until golden brown. Add the sherry, salt and pepper. Cook 1-2 minutes longer or until liquid is evaporated. Set aside and keep warm., Lightly oil the grill rack. Grill steak, covered, over medium heat or broil 4 in. from the heat for 5-6 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135degrees; medium, 140degrees; medium-well, 145degrees). Let stand for 5 minutes before slicing., Divide salad greens between two plates. Top with tomatoes, cucumber, olives, onion, steak and mushrooms. Sprinkle with cheese. Drizzle with vinaigrette. Serve immediately.