Trim fat from meat. If necessary, cut brisket to fit into a 3-1/2- or 4-quart slow cooker. Place onions in the cooker. Top with brisket. In a medium bowl stir together beer, chili sauce, and steak seasoning. Pour over onions and meat in cooker.
Cover and cook on low-heat setting for 10 to 12 hours or on high-heat setting for 5 to 6 hours.
To serve, remove meat from cooking liquid. Thinly slice meat across the grain. Using a slotted spoon, remove the onions from the cooking liquid and place on top of the meat. Drizzle with some of the cooking liquid. If desired, serve sliced meat and onions on buns. Makes 9 to 12 servings.