Coat beef in the flour and seal on both sides in the oil and place into a deep casserole dish.
Half and finely slice one of the onions, and finely chop the other. Layer over the onions and the mushrooms.
Mix the beef stock, wine, peppercorns, Worcestershire sauce and mustard in a jug and pour over the top. The liquid must cover the beef; add more boiling water if needed.
Seal with dish with a few layers of aluminium foil and bake in the centre of the oven for 3 hours.
Drain the liquid from the dish and thicken with Bisto to make the gravy. Return the gravy to the dish and cook for a further 15-20 minutes.
Serve with roast potatoes and seasonal vegetables. Leftovers reheat very well, simply wrap in foil again and bake in the oven for 30-45 minutes.