Season the brisket on all sides with salt and pepper then place in a glass baking dish. Cover with a layer of sliced onions. In a medium bowl, mix together the beer, chilli sauce and brown sugar. Pour over the roast. Cover the dish tightly with aluminium foil.
Bake for 3 hours in the preheated oven. Remove the aluminium foil then bake for an additional 30 minutes. Let the brisket rest and cool slightly before slicing and returning to the dish. Reheat in the oven with the sauce spooned over the sliced meat.