Cooking method:

1. Mix the lamb with the rosemary, cheese and ¼ tsp fine salt. Divide into 4 and roll into balls. Pat the balls between your hands and flatten slightly to around 1.5 – 2cm thick. Lay on a plate, cover and leave at room temperature while you prepare for others.
2. Sauté the onions with ½ tsp salt and the oil over medium low heat until caramelized – stirring frequently to prevent them from sticking to the pan. Add the vinegar and cook 1 minute, stirring. Keep warm.
3. Mix the mayonnaise with the horseradish and beetroot, adding salt to taste.
4. Cook the burgers over medium high heat until browned, then turn over and cook on the other side for 2 minutes at which point the meat will be medium. Take off the grill and keep warm – ideally rest for 5 minutes.
5. Grill the buns on their cut side in the same skillet until warmed through. The lamb juices will be absorbed by the buns which will make them even tastier.
6. To serve, spread the beetroot mayonnaise on the bottom of the buns. Lay the watercress and lamb on the burger. Spoon on the caramelised onions and put the bun on top.