Preheat the oven to 180°C (gas mark 4). Weigh the joint of beef and calculate the cooking time.
Put the meat, fat side uppermost, on a rack in a roasting tin. Season with pepper, then spread with 3 tsp of the mustard, if using. Roast the meat in the oven for the calculated time, basting occasionally with the juices in the tin.
Meanwhile, prepare the Yorkshire pudding batter. In a bowl, mix together the flour and a pinch of salt. Make a well in the centre and add the egg. Add a little of the milk and beat together, gradually beginning to work in the flour. Slowly beat in the remaining milk and 4 tbsp water, until all the flour is incorporated and the batter is smooth. Set aside.
Next prepare the vegetables. Put the potatoes in a large saucepan of boiling water and boil for 5 minutes. Drain well and return to the pan, then cover and shake vigorously to roughen the surface of the potatoes (this helps to make them crisp).
Put the parsnips and carrots in another large saucepan of boiling water and boil for 3 minutes, then drain.
One hour before the end of the roasting time for the beef, put 2 tbsp of the oil in a non-stick roasting tin and the remaining 1 tbsp oil in another non-stick roasting tin. Heat on top of the cooker, then add the potatoes to the 2 tbsp oil and the parsnips and carrots to the 1 tbsp oil. Baste each piece of vegetable with oil, then quickly place the tins in the oven with the beef. (With a gas oven, put the potatoes above the meat and the vegetables below.) After 30 minutes, turn the potatoes and vegetables so they will crisp and brown evenly.
When the beef is cooked, remove it from the oven and increase the heat to 220°C (425°F, gas mark 7). Place the beef on a warmed plate, cover with foil and keep warm. Divide the oil for the Yorkshire puddings among 12 non-stick patty tins and put in the top of the oven to heat for 2–3 minutes. (Move the potatoes down a shelf.) Stir the batter, pour it into the tins and bake for 15 minutes or until the puddings are risen and golden brown.
Meanwhile, steam the broccoli for 10 minutes or until tender, and make the gravy: pour the fat very slowly from the roasting tin, leaving the sediment behind. Place the tin on top of the cooker and pour in the stock. Bring to the boil, stirring and scraping up all the browned cooking residue on the bottom of the tin, then simmer until slightly reduced. Season and stir in the remaining mustard, if using.
To serve, transfer the meat to a warmed serving platter and surround with the roast potatoes and root vegetables and the Yorkshire puddings. Put the broccoli in a warmed serving dish. Add any meat juices that have collected on the plate to the gravy and stir to combine, then skim off any fat. Pour the gravy into a gravy boat and serve immediately.
