Place the mushrooms, onions, rosemary and chicken into a 3 1/2-quart slow cooker.
Stir the cornstarch, soup and wine in a small bowl. Pour over the chicken and vegetables.
Cover and cook on LOW for 8 to 9 hours*. Remove and discard the rosemary. Serve the chicken mixture with the mashed potatoes.
*Or on HIGH for 4 to 5 hours.