Harissa-spiced lamb burgers

heat a little oil in a small pan, add the onion and cook for 5 minutes until softened and golden then remove from the heat. In a large bowl, mix together the lamb mince, harissa, mint, and cooked onion. Form into 4 burgers and chill for 15 minutes. heat the remaining oil in a non stick frying pan or griddle pan over a medium heat. Cook the burgers for 5-6 minutes on each side until nicely browned and cooked through (or on a barbecue). Meanwhile lightly toast the cut side of the rolls on the frying pan or griddle. To serve, place a handful of rocket on the bottom half of each roll, top with a burger, a large dollop of tzatziki and the remaining half roll. Serve hot.