Heat the oven to 200c/fan 180c/gas 6. Put onions in a roasting tray with 4 tbsp water. Sit a 1.75k the leg of lamb on top and rub the with oil and then honey. stab the skin side of the lamb 15 times with a small, sharp knife, twisting to make small holes. Stuff a handful of small rosemary sprigs into the holes. Season well.