Put the beef on a board and season all over with salt and pepper. Be generous!
Heat the oil in a frying pan, then brown the beef on all sides. Transfer to a roasting tin.
Roast for 20 minutes per 500g for medium (still pink in the middle).
Meanwhile, Cook the Yorkshires, roast potatoes and parsnips in the oven at the same time, following the packet instructions.
Remove the beef from the oven, cover with foil and leave to rest.
Heat the butter in a pan until melted and fry the leek and mushrooms until the leeks have softened and the mushrooms are turning golden, 5 to 10 minutes. Add the hot beef stock and simmer for 2 to 3 minutes. Stir in the cream and season, allowing the sauce to simmer for a minute or two more. Pour any juices from the beef into the sauce.
Slice the beef and serve with the roast potatoes, Yorkshire puddings and spoon over the creamy leek and mushrooms.
This joint of beef leaves plenty of leftovers for other meals, saving you time and money.