Preheat the oven to 160ºC/325ºF/gas 3. Place a large shallow casserole pan on a high heat, with a large non-stick frying pan on a high heat alongside. Dice the beef into 5cm chunks and toss with a generous amount of black pepper and a pinch of sea salt, then dry fry in the hot frying pan with the bone for 8 minutes.
Meanwhile, wash and trim the carrots, chop into 5cm chunks, and place in the casserole pan with 1 tablespoon of olive oil. Peel and quarter the onions and break apart into petals, straight into the pan, stirring regularly. When the meat is nicely coloured, tip it into the casserole pan, then stir in the Worcestershire sauce and mustard, and cover with 800ml of boiling kettle water.
Cover, then cook in the oven for 4 hours, or until the beef is meltingly tender. Loosen with a splash of water, if needed. Taste, season to perfection, and serve.