- Preheat the oven to 190°C/gas 5. Mix the breadcrumbs and fennel seeds in a bowl with the leaves from 2 of the thyme sprigs and seasoning to taste. Sprinkle 2 tbsp of the olive oil and the milk over the crumbs and mix thoroughly with a spoon until they are moist and clump together.
- Place the beef in a large, lightly oiled roasting tin and spread the meat with the mustard. Press the crumb mixture on the top of the meat, pressing it firmly all over with both hands to make a neat, firm crust. Roast for about 1¾ hours for a medium result or about 2 hours for well-cooked but juicy meat. Cover the crust loosely with a piece of foil after 1 hour, to prevent over-browning, then remove the foil for the final 5 minutes of cooking.
- Meanwhile, cut the celeriac and sweet potatoes into 3.5cm chunks and place in a large bowl. Halve the carrots lengthways, then cut them across into chunky sticks. Add to the bowl with the shallots and the remaining thyme leaves and oil. Mix well.
- The vegetables require about 1 hour for roasting, so put them in the oven when the beef has been cooking for 45 minutes to 1 hour. Arrange them around the meat, placing any that won't fit in a separate roasting tin or large ovenproof dish, if necessary. Turn the vegetables and baste them once during cooking.
- Remove the meat and vegetables to a warmed serving dish and cover with foil. Pour 300ml water into the roasting tin and bring to the boil on the hob, stirring and scraping up all the browned cooking residue on the bottom of the tin. Add the sherry, Madeira or port and boil steadily for 5–7 minutes, stirring from time to time, until the liquid is reduced by about half. Season to taste, then strain into a jug.
- Slice the meat and serve with the roast vegetables and gravy. Any pieces of crust that break off as the meat is sliced can be divided among the plates.
*If liked, add some finely chopped garlic to the crumb mixture. Fresh oregano could be used instead of thyme. *Spread horseradish sauce over the meat in place of the mustard. *Use red wine instead of a mixture of water and sherry, Madeira or port to deglaze the roasting tin.
Calculate the cooking time for the beef at 55 minutes per 1kg, plus 45 minutes for medium, or 65 minutes per 1kg, plus 45 minutes for well-done meat that's still juicy.