Mix together the oil, garlic, and thyme and season with salt and freshly ground black pepper. Brush the mixture all over the beef, ensuring it is thoroughly coated. Preheat a covered barbecue to medium indirect heat. Put the beef in a baking dish and place it on the rack in the middle of the barbecue. Lower the lid and cook for 40 minutes for medium rare. Remove the beef from the heat, cover loosely with foil, and leave to rest until ready to serve.

While the beef is cooking, caramelize the onions. Heat the oil in a saucepan over medium heat. Add the onions and cook for 2–3 minutes, or until slightly softened. Tum the heat down to low and cook the onions for another 15 minutes, stirring occasionally, until they start to caramelize. Stir in the sugar and cook for a further 5 minutes, or until caramelized.

To make the horseradish cream, dry the horseradish thoroughly and grate very finely. Mix with the sugar, mustard, and vinegar. Whip the cream until it forms stiff peaks. Fold in the other ingredients and season to taste with salt and freshly ground black pepper.

Once the beef has rested, cut it into ½-inch-thick slices. Scatter with extra thyme sprigs and serve with the horseradish cream and caramelized onion. Serve with steamed potatoes and green beans.