Arrange sweet onions in the bottom of a slow cooker.
Mix salt, brown sugar, paprika, black pepper, garlic powder and chilli flakes together in a bowl until seasonings are well mixed; rub all over pork shoulder. Place pork in the slow cooker, fat-side up.
Cook on Low for 1 hour. Pour ginger beer into slow cooker. Continue cooking until pork is tender and pulls apart easily with a fork, 9 to 11 hours.