HEAT dressing in large skillet on medium-high heat. Add chicken, in batches; cook 2 to 3 min. on each side or until evenly browned on all sides.
PLACE mushrooms, onions, peppers and garlic in slow cooker; top with chicken. Combine tomatoes and thyme; pour over ingredients in slow cooker. Cover with lid. Cook on LOW 6 to 8 hours (or on HIGH 5 hours).
SERVE chicken mixture over rice.