Slow cook the lamb: Add the lamb shoulder to a slow cooker. If your lamb roast has a fat cap, make sure the fat cap is at the top, facing you, so all the juices melt and flow down through the meat as the lamb slow cooks. Pour the beef stock and aged balsamic vinegar over top. Arrange the sliced onion around the lamb roast. Top with the garlic, rosemary, thyme, Kosher salt, and ground black pepper. Slow cook on low for 8 hours, tossing the lamb every so often if you’re around, until the lamb shreds easily with a fork.