- Take the beef out of the fridge and allow it to come up to room temperature (about 1 hour).
- Preheat the oven to 230°C/Gas 8.
For the stuffing
- Place a heavy-based sauté pan over medium heat and add a dash of oil.
- When hot, add the onion with a pinch of salt and pepper and sauté until softened.
- Add the mushrooms and cook until tender. Add the garlic and sauté for 2 minutes, then add the spinach, in batches if necessary.
- Once the leaves have completely wilted, remove the pan from the heat and finely chop.
- Transfer to a bowl and allow to cool slightly.
- Add the breadcrumbs and herbs to the stuffing and stir well.
- Taste and adjust the seasoning as necessary.
- Stir in the egg yolk to bind the mixture.
For the ribs
- Place the beef on a chopping board with the ribs on the right-hand side.
- Insert the tip of a sharp knife on the left-hand side of the meat about 3cm in from the edge and make a long incision just above the ribs, going two-thirds of the way into the joint. The idea is to separate the meat from the bones, but leave it attached along the bottom.
- Gently pull the meat apart and push the stuffing into the gap (it might not take all the stuffing, in which case, put the excess in a small ovenproof dish, cover with a lid or foil, and bake it for the last 10-15 minutes of the meat cooking time).
- Tie a couple of pieces of string horizontally around the meat and vertically between the rib bones to secure the stuffing in place.
- Put the mustard powder in a bowl with a pinch of salt and pepper and enough olive oil to form a thin paste. Rub the mixture into the meat.
- Place the carrots and parsnips into the roasting tin and drizzle with olive oil.
- Put the meat on top of the vegetables and roast for 15 minutes to brown the outside of the meat.
- Lower the temperature to 180°C/Gas 4 and continue roasting for 15 minutes per 450g of meat, plus 15 extra minutes at the end if you like rare beef (the internal temperature should register 50-53°C on a meat thermometer) or for a further 20-30 minutes if you prefer beef medium rare (internal temperature 54-57°C).
- Remove the vegetables after 35 minutes and set aside ready to reheat before serving.
- Once cooked, remove the beef from the oven, cover loosely with foil and leave to rest for at least 20 minutes, and anything up to 45 minutes.
- Place the vegetables back in the oven at 180°C for 15 minutes to finish cooking and reheat.
- Remove the string from the beef and slice away the rib bones, then carve the meat and spoon the stuffing and vegetables on to a serving platter.
- Serve with Yorkshire puddings using this recipe.