Trim fat from lamb.
Place lamb in a large heavy-duty, zip-top plastic bag; pour vinaigrette over chops. Seal bag, and marinate in refrigerator 8 hours, turning bag occasionally.
Pour oil into a large nonstick skillet; place skillet over medium heat until hot. Add onion; cover and cook 20 minutes or until golden, stirring often. Add brown sugar and balsamic vinegar; cook 2 minutes, stirring constantly. Add water; bring to a boil, scraping browned bits. Add Swiss chard; cover and cook 3 minutes or just until wilted, stirring once. Set aside, cover, and keep warm.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Remove lamb from bag, reserving marinade. Place lamb on rack; grill, uncovered, 3 to 5 minutes on each side or until desired degree of doneness, turning occasionally and basting with reserved marinade. Serve with onion mixture.