Cut brisket in half; rub with liquid smoke if desired, celery salt, pepper and salt. Place brisket fatty side up in a 6-qt. electric pressure cooker. Top with onion. Combine beer and Worcestershire sauce; pour over meat. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 70 minutes. Allow pressure to naturally release for 10 minutes; quick-release any remaining pressure. If brisket isn't fork-tender, reseal cooker and cook for an additional 10-15 minutes. , Remove brisket, cover with foil and keep warm. Strain cooking juices, then return juices to pressure cooker. Select saute setting and adjust for medium heat; bring liquid to a boil. In a small bowl, mix cornstarch and water until smooth; gradually stir into juices. Cook and stir until sauce is thickened, about 2 minutes. Serve sauce with the beef.