Heat the oil in a large skillet or sauté pan over high heat. Season all sides of the ribs with salt and pepper. Cook them until a rich brown crust develops on the outside. Remove the ribs and place in a slow cooker. While the pan is still hot, add the beer and scrape up any bits stuck to the bottom with a wooden spoon. Pour the beer over the short ribs. Add the broth, carrots, onions, celery, garlic, and bay leaves to the short ribs and set the slow cooker to high. Cook for 4 hours, until the beef is tender and nearly falling apart. Discard the bay leaves. If using gremolata, mix the parsley, garlic, and orange zest. Serve the beef (along with some of the reduced sauce) over soft polenta or mashed potatoes. Sprinkle with the gremolata (if using).