Preheat oven to 450. Pour Guinness into bowl and set aside so beer flattens. Rub roast with prime rib rub seasoning. In large pan over medium high heat add vegetable oil. Sear roast on each side until browned. Immediately remove meat from pan and set in large roasting pan. Coarsely chop onions and add to pan along with garlic. Add stock and browning liquid. Roast in preheated oven 1 hour. Reduce heat to 350 and add Guinness. Liquid level should be at least halfway up side of roast. Roast an additional 2 hours checking liquid level periodically and adding water if needed.