1. Sprinkle steaks with black pepper. Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add steaks; cook 2 minutes on each side. Reduce heat to medium; cook 3 additional minutes, turning once.
2. Remove steaks from skillet; spread mustard over steaks, and top with Worcestershire sauce. Set aside, and keep warm.
3. Coat skillet with cooking spray; add onion, and cook, stirring constantly, 2 minutes. Add mushrooms and remaining 3 ingredients; cook, stirring constantly, 5 minutes.
4. Return steaks to skillet. Cook 4 to 5 minutes or to desired degree of doneness. Transfer steaks to 4 individual serving plates; spoon mushroom mixture over steaks.