Slice the onions into rings and dice the carrots. Cut the steak in large chunks about 5cm/2in across and toss in the seasoned flour. Heat the oil and butter in a large casserole dish, add the onions and cook for 5 minutes until browned. Add the steak and brown all over. Add the carrots, then gradually stir in the Guinness, stock, purée, mustard, bouquet garni, sugar, salt and pepper. Bring to the boil, stir well, then cover tightly with a lid and simmer gently for 1½ hours, stirring occasionally. Add the prunes and simmer for a further ½-1 hour until the meat is tender. Serve sprinkled with parsley.