Sprinkle steak evenly with salt and pepper. Place steak on a broiler pan coated with cooking spray. Broil 6 minutes on each side or until desired degree of doneness; let stand 10 minutes.
Heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add shallots to pan; cook 3 minutes, stirring occasionally. Add mushrooms to pan. Cover and cook 4 minutes or until mushrooms are tender, stirring occasionally. Stir in 1/3 cup water, mustard, worcestershire, and thyme. Cover and cook 2 minutes. Remove from heat; stir in sour cream.
Cut steak diagonally across grain into thin slices. Serve with mushroom sauce.