1. Preheat the oven to 220 degC. Pat the beef dry and tie tightly at regular 5cm intervals so it will hold its shape.
2. Mix the oil, 2 tablespoons of the mustard, the thyme, salt and a little pepper in a bowl. Spread the mixture over the underside of the meat, keeping a little to spread over the fat on top.
3. Place the beef in a medium-sized roasting pan and place in the oven. After 10 minutes turn the oven down to 200°C and roast the beef for a further 20-30 minutes, depending on how rare you like it.
4. Remove the beef from the oven, place on a heated plate, cover loosely with foil and a large towel and let it rest for at least 20 minutes.
5.  Meanwhile, make the gravy. Place the roasting pan over a gentle heat and add the flour. Stir vigorously with a wooden spoon to loosen any pan drippings and let the flour brown slightly in the leftover fat. Stir in the remaining mustard and mix well. Add the stock and stir until the gravy comes to the boil. Allow to bubble gently until ready to serve.
6. To serve, carve the meat into neat slices, place on a heated serving platter and pour a little gravy over. Hand the rest around in a jug.

Tip:  For horseradish mustard,I use one from www.mustardmakers.co.nz