MELT butter in large skillet on medium-high heat. Add chicken; cook 6 min. Turn. Add onions and garlic; cook 6 min. or until chicken is done (165ºF), stirring onions occasionally.
SPOON rice onto platter; top with chicken.
MICROWAVE sour cream in microwaveable bowl on HIGH 30 sec. or until heated through; stir. Spoon over chicken.