Generously season the brisket with dry rub seasoning on all sides. Place in a baking dish. Cover the beef with the sliced onions.
Combine the beer, barbecue sauce, brown sugar, canned tomatoes and chilies. Pour over the beef and cover the dish with foil. Bake for 3 hours. Leave the oven on.
Remove the foil, add the potatoes and cook, uncovered for another 40-45 minutes or until the potatoes are tender. Let rest for 10 minutes, slice the beef and return to the dish. Spoon the sauce over the beef and serve.