Preheat oven to 350° F. In a plastic bag, combine flour, salt and pepper. Coat the ribs with flour mixture, a few pieces at a time.
Heat oil in a Dutch oven. Brown ribs on all sides. Add onion and beer.
Bake for 1 hour. Remove ribs from the Dutch oven and pour off the liquid. Combine ketchup, Worcestershire sauce, vinegar, garlic and cayenne pepper. Place ribs back into the Dutch oven. Pour sauce over ribs and cook another 1/2 hour.