Gather and measure all the ingredients so they are ready to use. Preheat your oven to 325°F (165°C), allowing it to come fully up to temperature while you prepare the brisket.

Season the trimmed beef brisket generously on all sides with salt and pepper, pressing the seasoning lightly into the meat so it adheres. Place the brisket into a glass baking dish, positioning it fat side up if there is any remaining fat. Arrange the thinly sliced onion evenly over the top of the meat to form a flavorful layer.

In a medium mixing bowl, combine the beer, tomato-based chili sauce, and brown sugar. Stir until the sugar is mostly dissolved and the mixture looks smooth and unified. Pour this sauce carefully over the brisket and onions, making sure the meat is well coated and some of the liquid surrounds it in the dish. Cover the baking dish tightly with aluminum foil, crimping the edges to seal in the steam.

Transfer the covered dish to the preheated oven and bake for about 3 hours, allowing the brisket to braise slowly until it becomes very tender. After this time, remove the foil and return the dish to the oven. Continue baking for an additional 30 minutes so the sauce can reduce slightly and the top of the brisket can take on a richer color.

Take the dish out of the oven and let the brisket rest for a short while so the juices can settle. Move the brisket to a cutting board and slice it thinly against the grain. Return the sliced meat to the baking dish, nestling it back into the sauce. Spoon some of the sauce over the top, then reheat briefly in the oven if needed before serving, ensuring the brisket is piping hot and well coated in its sweet and tangy cooking juices.