In a casserole dish brown the beef in sunflower oil. You may need to do this in batches. Remove when browned and set aside.
Lower the heat and add a little more oil to the casserole then add the onions, carrots, celery, garlic, bay and thyme and allow to cook for 5 minutes. Add the flour, stirring well, then allow to cook for 2 minutes stirring all the time. Pour in the Guinness® and bring the casserole to a simmer to thicken the sauce. If it looks a bit too thick add some water or beef stock.
Put the beef back in and stir. Put a lid on the casserole and cook in the oven for a minimum of 2 hours. You can leave the casserole cooking for longer just use the lowest oven shelf and it will be great after 4 or 5 hours.
Season with salt and pepper and serve. Great with mashed potato to soak up the sauce.