Trim the meat of any fat or gristle, cut into cubes of 5cm and toss them in a bowl with 1 tablespoon oil. Season the flour with salt, freshly ground pepper and a pinch or two of cayenne. Toss the meat in this mixture.
Heat the remaining oil or dripping in a wide frying pan over a high heat. Brown the meat on all sides. Add the onions, crushed garlic (if using) and tomato purée to the pan, cover and cook gently for about 5 minutes.
Transfer the contents of the pan to a casserole, and pour some of the Guinness into the frying pan. Bring to the boil and stir to dissolve the caramelized meat juices in the pan. Pour on to the meat with the remaining Guinness; add the carrots and the thyme. Stir, taste, and add a little more salt if necessary.
Cover with the lid of the casserole and simmer very gently until the meat is tender - 2 to 3 hours. The stew may be cooked on top of the stove or in a low oven at 150C/gas mark 2. Taste and correct the seasoning.
Scatter with lots of chopped parsley and serve with champ, colcannon or plain boiled potatoes.