Combine first 4 ingredients in a bowl; set aside.
Place a large nonstick skillet over medium-high heat. Coat pan with cooking spray; add onion, and sauté, 4 minutes or until tender. Add garlic; cook 30 seconds. Remove onion mixture from pan.
Recoat pan with cooking spray; add chicken, and brown well on all sides. Add onion mixture, and drizzle evenly with balsamic mixture. Cover, reduce heat, and simmer 15 minutes. Turn chicken; cover and simmer 15 minutes or until chicken is done.