Season the beef brisket with salt and place in a baking dish. Cover the entire piece of meat with onion slices.
In a medium bowl mix together the beer, brown sugar, beef stock cube, pepper, garlic, bay leaf and thyme. Pour over the roast. Cover with aluminium foil.
Bake for 4 hours in the preheated oven. The brisket should be fork-tender. Mix together the cornflour and water then stir into the juices in the baking dish to thicken. Remove the bay leaf. Slice and serve the meat.