Put onions into the crockery insert of a 4 to 7 quart slow cooker. Cut the brisket into 2 or 3 pieces and place on the onions. Sprinkle with the pepper.
Combine the beer, ketchup, brown sugar, Worcestershire sauce, and garlic; pour over the meat. Cover and cook on HIGH for 4 to 6 hours, or cook on LOW for 8 to 10 hours.
Remove the beef to a serving platter.
Add the flour-water mixture to the juices in the slow cooker. Cook, uncovered, on HIGH for about 5 to 8 minutes, or until thickened. Serve the thickened sauces with the brisket.
Calories per serving: 471
Fat per serving: 14.2