Preheat heat oven to 170°C / 325°F / Gas Mark 3. Cut the steak into 4cm pieces and toss lightly in the seasoned flour. Heat a third of the butter and 1 tbsp of oil in a frying pan and brown the meat briskly in batches, without overcrowding the pan. Transfer the browned beef to the casserole.
Add a little more butter and oil to the pan and fry the onions until lightly browned. Spoon into the casserole with the mushrooms and carrots.
Add the Guinness and tomato puree and top up with hot water so it comes about 3/4 of the way up the meat. Add the bay leaf, salt and pepper. Bring to the boil, stir well, then cover tightly with a lid and simmer gently for 1 hour stirring occasionally, add the prunes and return to the oven for a further 30 minutes.