Preheat oven to 350F. Combine beer, cranberry sauce and catsup in medium bowl and set aside. Heat olive oil in heavy large Dutch oven over high heat. Season brisket with salt and pepper. Add brisket to Dutch oven and sear until brown, about 5 minutes per side. Transfer brisket to plate. Add sliced onion to same pot and sauté until soft and brown, stirring and scraping bottom of pot frequently, about 8 minutes. Place brisket on onions. Pour beer mixture over brisket. Bring liquids to boil. Cover pot tightly. Transfer to oven and bake until brisket is tender, about 3 hours. Let brisket cool 30 minutes before serving. Spoon fat from top of gravy. Transfer brisket to cutting board. Thinly slice brisket across grain. Return brisket to gravy.