Coat a large nonstick skillet with cooking spray; add margarine, and place over medium heat until margarine melts.
Add steak, and cook 6 minutes on each side or until desired degree of doneness.
Remove steak from pan; transfer to a serving platter, and keep warm.
Combine yogurt, mustard, and Worcestershire sauce in a small saucepan. Cook over low heat, stirring constantly, until thoroughly heated (do not boil). Spoon sauce over steak.
Sprinkle with parsley. If desired, garnish with rosemary sprigs and lemon wedges.