Trim the excess fat from the lamb. Combine the salt, brown sugar, pepper, garlic powder, dry mustard and cumin. Rub all over the lamb. Refrigerate overnight.
The next day, heat the grill to 300F with an indirect heat zone.
Place the onions in a disposable aluminum pan. Add a half a bottle of beer. Place the lamb on top of the onions. Place on the smoker; close the lid.
After the first hour and every 30 minutes after that, mop the lamb with the juices in the pan.
In about 2 hours, once the lamb has a mahogany color, cover the pan with foil. Continue cooking, until the internal temperature of the lamb reaches 200F, about 45 minutes.
Separate the onions from the pan and reserve. Pull the lamb and toss with the juices.
Build your sliders on toasted buns with pulled lamb, braised onions and shredded cabbage. Top with Carolina mustard BBQ sauce.