Place brisket fat-side up in a large stockpot, along with spice packet and any juices remaining in packaging. Add beer, onion, celery, carrots, garlic, peppercorns, bay leaves, and water to cover; bring to a low boil over medium-low heat. Cover and simmer for 3 1/2 hours. Remove from heat; and let meat rest in cooking liquid, about 20 minutes more.
Transfer brisket to platter; cut into thin slices across the grain.