Place the thyme, paprika and mustard powder in a bowl with a teaspoon each of salt and pepper, then mix to combine. Wipe the meat with damp kitchen paper and then spread a thin layer of the mustard all over the fat side of the joint. Sprinkle the spice powder on top, patting it down gently to help it stick.
Preheat the oven to Gas Mark 7, 220ºC (425ºF). Pour the olive oil into roasting tin and heat in the oven for 5 minutes. Add the onion and carrot, tossing to coat. Season with salt and black pepper. Sit the beef on the bed of vegetables. Place the roasting tin in the pre-heated oven.
Reduce the oven temperature to Gas mark 5,190ºC (375ºF). Roast the beef for 1 hour and 15 minutes for rare, an extra 15 minutes for medium-rare and an extra 30 minutes for well done.
Remove the beef from the tin and place on a large dish. Rest in a warm place for at least 20 minutes before carving. To make the gravy, stir the flour into the juices in the roasting tin and then gradually stir in the stock. Place directly on the hob to heat and simmer for 5 minutes, stirring and scraping the bottom with a wooden spoon to release any sediment. Season and pour through a sieve into a gravy boat, discarding the vegetables that the beef has been roasted on. Carve the beef into slices and arrange on warmed plates with a dollop of creamed horseradish and the roasted root vegetables. Hand round the gravy separately.