Combine the spices in a small bowl, then rub liberally over the pork shoulder. Place the shoulder and onion in a slow cooker, and pour in the stock.
Cook on low for 8-12 hours, or until the meat is ultra-tender and falling apart.
Remove the meat from the slow cooker and allow it to cool long enough for you to comfortably handle it.
Separate the meat from the fat and bones, and then shred it.
TIP: I have found using the paddle attachment of my KitchenAid mixer makes shredding a breeze. Add several cups of meat chunks to the bowl, then beat on low for 30-60 seconds until it is shredded. Much easier than the two-fork method.