Preheat the oven to 190°C. Mix the breadcrumbs and fennel seeds in a bowl with the leaves from two of the thyme sprigs and seasoning to taste. Sprinkle 2 tablespoons 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 1 ¾ 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 overbrowning, then remove the foil for the final 5 minutes of cooking.
Meanwhile, cut the celeriac and sweet potatoes into 3.5 cm chunks and place in a large bowl. Halve the carrots lengthways, then cut them across into chunky sticks. Add to the bowl along 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 the vegetables around the meat, placing any that won't fit in a separate roasting tin or large ovenproof dish, if necessary. Turn the vegetables and liberally baste them once during cooking.
Remove the meat and vegetables to a warmed serving dish and cover with foil. Pour the water into the roasting tin and bring to the boil on the stove, 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 the gravy 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.