- 1 tbsp vegetable oil
- 1kg/2lb 2oz sirloin of beef, on the bone
- 2 tbsp English mustard powder
- 1 tbsp ground sea salt
- 1 tbsp cracked black pepper
- 2 tbsp olive oil
- 2 tbsp butter
- 1kg/2lb 2oz onions, peeled, sliced
- 4 tbsp freshly grated horseradish
- 1 tbsp white wine vinegar
- pinch English mustard powder
- 1 tsp caster sugar
- 150ml/5fl oz double cream
- 12-16 bread rolls, split in half
- 4 Little Gem lettuces, leaves separated
- For the roast beef, preheat the oven to 220C/425F/Gas 7.
- Heat the vegetable oil in a roasting tin set on the hob over a high heat, then add the sirloin and sear on all sides for 3-4 minutes, or until browned all over. Set aside.
- Mix together the mustard powder, sea salt and cracked black pepper in a small bowl. Rub the mixture all over the sirloin.
- Roast the beef in the oven for 30 minutes (for medium; reduce the time by ten minutes for rare, or increase by ten for well done). To test how done the meat is, pierce the centre of the sirloin with a meat thermometer and check the temperature. The temperature should be around 45C/115F for rare, 60C/140F for medium and 72C/160F for well done. The beef will continue to cook after it's been taken out of the oven, so allow about 10C/50F under the desired internal temperature before removing it.
- Remove the beef from the oven and set aside to rest somewhere warm for 15 minutes.
- Meanwhile, heat the butter and the olive oil together in a large frying pan. Add the onions and fry for 8-10 minutes over a medium heat, or until the onions are golden-brown. Set aside.
- For the creamed horseradish, mix all of the horseradish ingredients together in a bowl until well combined and slightly thickened.
- To serve, thickly slice the beef. Dip the cut sides of the bread rolls into the pan juices, then arrange a few slices of beef on one half. Top with the fried onions and a few leaves of Little Gem lettuce. Spread the creamed horseradish over the other half of the bun, then sandwich the two halves together and serve.