Trim fat from meat. If necessary, cut meat to fit into a 3-1/2- or 4-quart slow cooker. Place meat and onion in cooker. Add bay leaves, salt, garlic powder, and cloves; pour vinegar over meat in cooker.
Cover and cook on low-heat setting for 11 to 12 hours or on high-heat setting for 5-1/2 to 6 hours. Remove meat from cooker. Discard bay leaves. Using 2 forks, pull meat apart into shreds, discarding any bones and fat. Strain juices; skim off fat.
If desired, line rolls with spinach leaves. Place shredded meat on rolls. Drizzle meat with some of the strained juices. Serve remaining juices with sandwiches for dipping. Makes 8 sandwiches.