Sprinkle steaks with pepper and salt. Place steaks on a broiler pan coated with cooking spray; broil 5 minutes on each side or to desired degree of doneness.
Heat a nonstick skillet over medium-high heat. Coat shallots with cooking spray, and add to pan; sauté 2 minutes. Add broth; bring to a boil. Cook, stirring frequently, 5 minutes. Stir in mustard. Spoon sauce over steaks. Garnish with thyme, if desired.