The first step is to sear the meat. Rub the pork with salt and pepper. Next, make sure the pan used for searing is really hot before you add the meat. Sear the meat on each side.
Set the pork aside and use the pan to cook the onion and garlic. Heat oil in the pan and add the onions and garlic until the onions are translucent.
Add pork, onions, and garlic to the slow cooker on a high heat.
Mix the stock, vinegar, Worcestershire sauce, and tomato paste together. Then, stir in the mustard, chili powder, and cumin together in a bowl. Then add this to the slow cooker.
Cover the slow cooker with a lid and cook until the pork is easily shredded. This will take approximately 8 hours on a low heat or 6 hours on a high heat.