1) For the raita: Combine the yoghurt, pomegranate treacle, cucumber, cumin and black pepper in a bowl. Season with salt, then cover and refrigerate for at least 30 mins. Add the chopped mint just before serving, if desired. Refrigerate until ready to serve.
2) Heat a griddle to medium heat.
3) Combine the spices in a small bowl. Stir in enough oil to make a paste and set aside. Brush the lamb leg with some canola oil and season top side of the meat with the curry rub. Let it sit at room temperature for 30 mins.
4) Season the lamb with salt, to taste. Put it on the griddle, spice side down, and cook until slightly charred and a crust as formed. At this point, flip the leg over, close the griddle cover and cook to a medium-rare doneness. Remove the meat from the griddle to a cutting board, loosely tent it with foil and let it rest for 15 mins. Thinly slice the lamb, against the grain, and arrange the meat on a serving platter. Serve with the raita.