Heat your oven to 220°C/200°C/Gas 7. Unwrap your beef. Pat dry with kitchen paper and let it get up to room temperature.
While the beef warms up, pick the thyme leaves off their woody stalks and pop the leaves in a bowl. Add 1 tbsp Dijon mustard and 2 tbsp horseradish mustard. Tip in the breadcrumbs.
Add 2 tbsp oil to the bowl with a good pinch of salt and pepper. Stir together to make a sticky paste. Rub the mustardy mixture all over the beef and pop it in a small roasting tin.
Slide the tin into the centre of the oven. Turn the heat down to 180°C/Fan 160°C/Gas 4. Roast for 45 mins for rare, 1 hr for medium and 1 hr 15 mins for well done.
Take the beef out of the oven and set it aside on a plate or board, loosely covered with foil, to rest for 30 mins before carving.