Place the pork shoulder and sliced onion in a slow cooker. Pour the ginger beer over the pork. Cover and cook on low for 8-10 hours until the pork is tender and shreds easily.
Once cooked, remove the pork from the slow cooker and shred it using two forks. Mix in 1 cup of BBQ sauce, adding more to taste. Set aside the shredded pork.
Layer half of the mac and cheese in a baking dish. Top with half of the shredded pork. Repeat the layers. Drizzle with additional BBQ sauce and bake for 10 mins.