In a 5-qt. slow cooker, combine the first 6 ingredients. Place mustard seed and garlic on a double thickness of cheesecloth; bring up corners of cloth and tie with kitchen string to form a bag. Place in slow cooker. Add chicken and broth. Cover and cook on low 5-6 hours, until meat is tender. , Discard spice bag. Remove chicken; cool slightly. Stir linguine into soup; cover and cook on high until tender, about 30 minutes. Cut chicken into pieces and return to soup; heat through. Sprinkle with coarsely ground pepper and additional parsley if desired.