HEAT dressing in large nonstick skillet on medium-high heat. Add chicken, in batches; cook 2 to 3 min. on each side or until each piece is browned on both sides.
PLACE mushrooms, onions, peppers and garlic in slow cooker; top with chicken. Mix tomatoes and thyme; pour over chicken. Cover with lid. Cook on LOW 6 to 8 hours (or on HIGH 5 hours).
COOK rice as directed on package, omitting salt. Serve chicken mixture over rice.