To a crockpot, add the seared beef and onions, potatoes, carrots, celery, beef broth, Worcestershire sauce, garlic, thyme, rosemary, bay leaves, 1/4 tsp salt, 1/4 tsp pepper and the bottle of Guinness. Cook on high for 4-5 hours or low for 8-9 hours. 10 minutes before it's done, mix 2 tbsp of cornstarch with 2 tbsp of water. Pour into the crockpot and let cook to thicken.