Take the joint out of the fridge 30 minutes before cooking to bring it up to room temperature. Preheat the oven to 230ºC/gas mark 8. Weigh the beef to calculate the exact cooking time – for medium, cook for 15 minutes per 450g plus an extra 20 minutes; for well done, cook for 20 minutes per 450g plus an extra 20 minutes.
Put the beef in a large roasting tin. Sift the flour and mustard together in a bowl, then use a tea-strainer or sieve to dust it over the joint. Season with freshly ground black pepper.
Roast the joint for 20 minutes, then reduce the oven temperature to 200ºC/gas mark 6 and cook for the remaining calculated cooking time. Baste the joint every 30 minutes with the pan juices.
An hour before the end of the cooking time, put the onions, shallots and garlic in the roasting tin around the meat. Scatter over the sprigs of thyme, reserving a few sprigs for garnish. Toss the vegetables in the pan juices to coat and drizzle them with a little sunflower oil if necessary.
When the beef is cooked to your liking, transfer it and the roasted vegetables to a serving platter and cover loosely with foil. Let the meat rest for 20 minutes. Carve and serve with the roasted onion, garlic and shallots, plus veggies and trimmings of your choice. Garnish with the reserved thyme.