Place steak in nonmetal dish or large resealable food storage plastic bag. Pour wine and salad dressing over steak; turn to coat. Cover dish or seal bag. Refrigerate at least 4 hours or overnight to marinate.
In medium bowl, combine all sauce ingredients; mix well. Cover; refrigerate until serving time.
Heat grill. When ready to grill, remove steak from marinade; discard marinade. Place steak on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 12 to 15 minutes or until of desired doneness, turning once. Spread both sides of bread slices with 2 tablespoons of the butter. To toast, place on grill during last 2 to 4 minutes of cooking time. Turn once.
Melt remaining tablespoon butter in small saucepan over medium heat. Add mushrooms; cook 2 to 3 minutes or until lightly browned, stirring frequently.
To serve, spread 1/4 of the sauce on one side of each toasted bread slice; place on serving plates. Cut beef into thin slices diagonally across grain; place over sauce. Top each serving with mushrooms. Sprinkle with parsley.