Preheat the oven to 350 degrees F (175 degrees C).
Season the beef brisket with salt, and place in a baking dish. Cover the entire roast with onion slices. In a medium bowl, mix together the beer, brown sugar, beef bouillon, pepper, garlic bay leaf and thyme. Pour over the roast. Cover with aluminum foil.
Bake for 4 hours in the preheated oven. The brisket should be fork-tender. Mix together the cornstarch and water; stir into the juices in the baking dish to thicken. Remove the bay leaf. Slice and serve the meat.