To season lamb: Place the leg of lamb, fat side down, on work surface. Cut the meat partially away from the bone on each side, leaving the bone attached to the bottom.
In a small bowl, combine the coriander, cumin, paprika, salt and pepper. Rub half of the spice mixture all over the inside of the meat.
Roll the meat back onto the bone tightly and tie at intervals with kitchen string. Rub the remaining spice mixture all over the lamb. Set lamb aside.
To prepare crust: In a food processor, pulse parsley leaves and garlic until coarsely chopped. Place in a bowl and add bread crumbs and 2 to 3 tablespoons of olive oil just to moisten. Do not over mix; mixture should be loose. Set aside.
To roast: Preheat the oven to 200°C. Add water to the roasting pan. Place the seasoned lamb on a rack, fat side up, and place in the roasting pan with the water. Place in the oven and roast for 20 minutes or until it starts to brown on all sides.
Carefully turn lamb over and pat the crumb mixture on top. Some will fall to the bottom of the pan.
Reduce temperature to 175°C and continue cooking for another 1 to 1½ hours for medium rare, or until thermometer registers 55°C in the thickest part of the leg without touching the bone. Remove from the oven, tent with foil and allow it to rest for 10 minutes before carving.
To prepare sauce: In a food processor, puree all the sauce ingredients until almost smooth. The sauce should be the consistency of a thick vinaigrette. Taste and adjust seasoning as needed. The sauce should have a mild mint taste with a mild citrus finish.
To substitute with a boneless leg of lamb, brown lamb on all sides in a large skillet, about 15 minutes total, depending on size. Pat crust on top and transfer to a preheated 200°C oven. Cook for about 25 to 30 minutes for medium-rare (internal temperature of 50-55°C). Let rest before slicing.