Stud onions with cloves. Place in slow cooker stoneware and sprinkle with salt, peppercorns, thyme and orange zest. Pour orange juice and beef broth over onions, cover and cook on LOW for 8 hours or on High for 4 hours, until onions are tender.
Keep onions warm. In a saucepan over medium heat, reduce cooking liquid by half.
When ready to serve, cut onions into quarters. Place on a deep platter and cover with sauce. Sprinkle with parsley, if desired, and pass the hot pepper sauce, if desired.