Place brisket in a large roasting pan and pat dry with paper towels. Season generously on all sides with salt and the grilling rub. You want the rub to coat all sides of the brisket. Cover with plastic wrap and let stand at room temperature for 1 hour or refrigerate overnight.
Put the onions, garlic, stock and beer in a slow cooker. Add the brisket, fat side up. Cover and cook on high for 8 hours. Transfer the brisket to a carving board, cover loosely with aluminum foil and let stand for 30 minutes.
Slice the brisket across the grain, fill the rolls with a couple slices of meat and drizzle with bbq sauce. Pour barbecue sauce into a bowl and serve alongside.