Heat your oven to 220°C/Fan 200°C/Gas 7. Remove the beef from its wrapping. Pat dry with kitchen paper. Place in a small roasting tin and let it get to room temperature.
Spread the beef with 1 tsp of the Dijon mustard, coating the sides as well as the top. Season all over with salt and pepper.
Wrap the unpeeled garlic cloves in a small square of foil. Nestle the foil parcel next to the beef joint. Cover each cut-side of the beef with a square of foil to keep it moist, so only the fat is directly exposed to the heat.
Slide the beef into the middle shelf 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.
Remove the beef from the dish and rest on a plate, loosely covered with foil, in a warm place for at least 30 mins (longer if you can). Whip up your sides and sauce while it relaxes.
Remove the garlic from the foil and squeeze out the roasted flesh. Mash with the back of a fork and add to the beef juices in the roasting tin. Whisk in the remaining mustard. Pour in a mugful of water, beef stock, wine or beer. Put the dish on a medium heat and cook for 10 mins till bubbling.
Finely chop the thyme sprigs and add them to the sauce, keeping a few leaves back for garnishing, and cook for a further 10 mins. Taste and add more salt and pepper if it needs it. If you want a sweeter sauce, add a drizzle of honey.
For a thicker sauce, spoon 3 tbsp sauce into a jug. Slowly whisk in 1-2 tbsp plain flour to make a smooth paste. Swirl this into your sauce and cook till it thickens. Repeat till it’s as thick as you like it.
Carve your rested beef and pour over the thyme and mustard sauce. Scatter over the remaining thyme leaves to serve.