1Roughly chop the onions and smash the garlic cloves. Layer them on the bottom of your slow cooker.
2Place the pork shoulder on top of the onions and garlic.
3In a bowl, whisk together the ketchup, apple cider vinegar, brown sugar, Worcestershire sauce, smoked paprika, black pepper, salt, mustard powder, and cayenne if using.
4Pour the sauce over the pork, making sure to coat it well. The pork doesn't need to be submerged—the slow cooker will do the work.
5Cover and cook on low for 8 hours, or on high for 5-6 hours. The pork is done when it shreds easily with a fork.
6Using tongs, transfer the pork to a cutting board or large bowl. Discard any large bones if present.
7Shred the meat with two forks or shredding forks, working against the grain. The meat should fall apart with minimal effort.
8Skim any excess fat from the cooking liquid if you'd like. Pour the liquid back over the shredded pork and stir well. Let it sit for 5 minutes to soak up the sauce.
9Taste and adjust seasoning with salt, pepper, or more vinegar as needed. Serve on buns, in tacos, or however you like it.