In the slow cooker, combine the onions, garlic, ketchup, tomato paste, Worcestershire sauce, chili powder, balsamic vinegar, and honey. Season with salt and pepper, stir well, and cook on high for 1 hour.
Towards the end of the hour, heat a large pan over medium high heat. Add the oil. Season the lamb with salt and pepper. Sear the lamb on all sides, turning it, and allowing it to rest on the edge of the pan as needed. It’s ok if you can’t get the ends browned, but you want a nice sear on as much of the exterior as you can get. This should take about 10 minutes total.
Tuck the lamb into the sauce already heating in the slow cooker, and spoon some over the top. Reset the cooking time to 4 hours on high.
After 4 hours remove the lamb to a shallow dish and using two forks pull the lamb into small bite-sized pieces. You can make them as small or as large as you like, depending on preference and how you plan to serve the lamb.
Spoon some of the sauce over the lamb. If you want to serve it in sandwiches, pile it onto the buns, and spoon some of the Sriracha Mayo on top. You’ll likely have some leftovers, which is a very good thing.