Chicken Breast Halves should be boneless. Cook and stir onion in oil in a 10-inch skillet until tender. Stir in remaining ingredients except the chicken breasts and mushrooms.
Heat to boiling; reduce heat and simmer, uncovered, for 30 minutes. Place chicken, skin sides up, in a single layer in skillet. Cover and cook over medium-low heat for 30 minutes. Add mushrooms, cover and cook until chicken is done, about 15 minutes longer.