Take the meat out of the fridge 2 hours before you plan on cooking it to bring it to room temperature. Weigh the beef. Tie with string between the bones to keep the roast nicely shaped.

Preheat the oven to 230°C. To keep the bones white, wrap each one in foil.

Combine the oil, horseradish, horopito, Dijon, and thyme in a small bowl and smear over the beef. Put the onions and beef stock into a roasting dish and sit the beef on top.

Roast in the oven for 20 minutes, then lower the heat to 160°C. Roast the beef for a further 15 minutes per 500g for medium rare (20 minutes for medium), or until the internal temperature reaches 50°C (the temperature of the beef will rise when you take it out of the oven, taking it to medium rare). Approx 1 1⁄4 - 1 1⁄2 hours for a 3.5kg roast.

Remove from the oven, cover lightly with a sheet of baking paper or foil and a clean tea towel, and set aside to rest in a warm place for at least 30 minutes before removing the foil from the bones and serving with the smashed potatoes and gravy.