Chicken Breasts with Tomato and Olive Sauce

In 12-inch skillet over medium-high heat, in hot olive or salad oil, cook chicken-breast halves until golden and juices run clear when chicken is pierced with a knife, about 8 minutes. Remove chicken to plate. In drippings in skillet over medium heat, cook onion until tender-crisp. Add red wine vinegar and cook until onion is very tender; stir in chopped tomatoes and olives. Over high heat, heat to boiling. Return chicken to skillet; heat through.