Serves 4
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  1. Heat the butter, olive oil and sugar into a large frying pan over a medium heat. Add the onions in a single layer and cook for five minutes. Pour in the red wine vinegar, cover with a lid and reduce the heat to low. Cook for 25-30 minutes or until golden and caramelised, checking every few minutes and adding a splash of water if the pan becomes too dry.
  2. In a bowl, combine the mayonnaise with the horseradish sauce and set aside.
  3. Preheat the barbecue to a high heat.
  4. Cover the steaks with cling film and bash with a rolling pin until they are all roughly the same thickness. Season generously with salt and black pepper.
  5. Cook the steaks on the barbecue for 2-3 minutes per side or until cooked to your liking.
  6. Remove the steaks to a plate, cover with foil and allow to rest for 6-8 minutes. Slice against the grain in 1cm-thick slices. Stir the juices released from the steaks into the caramelised onions.
  7. Horizontally split open the pieces of baguette. Spread on some of the horseradish mayonnaise and layer over some rocket or watercress, the caramelised onions and the slices of steak. Sandwich together and press down lightly.

Ciabatta can be used as an alternative to rustic baguette, if you wish.
Watercress can be used as an alternative to rocket, if you wish.

Nutrition Facts

Per serving: 570kcals, 17.6g fat (5.8g saturated), 58.6g carbs, 21.1g sugars, 42.3g protein, 2.7g fibre, 0.486g sodium