Combine first 10 ingredients in a large zip-top plastic bag. Seal and marinate in refrigerator 30 minutes. Remove steaks and vegetables from bag, reserving marinade. Place reserved marinade in a small saucepan; bring to a boil. Cook until reduced to 3 tablespoons (about 2 minutes).
Place a grill pan coated with cooking spray over medium-high heat until hot. Add shallots and bell peppers; cook 3 minutes on each side or until crisp-tender. Chop shallots and bell peppers; place in a small bowl. Stir in reduced marinade; set aside.
Press 1 teaspoon coarsely ground pepper onto 1 side of each steak. Wipe pan clean with paper towels; recoat with cooking spray. Place pan over medium-high heat until hot. Add steaks, peppered sides down; cook 3 minutes on each side or until desired degree of doneness. Add bell pepper salsa; cook until thoroughly heated. Garnish with thyme sprigs, if desired.