Take the beef out of the fridge and allow it to come up to room temperature (about 1 hour). At that point preheat the oven to 230°C/Gas 8.
Meanwhile, make 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.
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. 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.
To serve, 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.