Add a long sheet of plastic wrap on a flat surface. Add the lamb to the center, fat side up.
Score the lamb in a diamond pattern on the top, just cutting through the fat layer.
In a medium-sized bowl stir together all of the remaining lamb ingredients.
Pour the mixture over the lamb. Gather up the plastic wrap and wrap the lamb tightly, using additional plastic wrap if needed.
Refrigerate for 1 hour, and up to 24 hours.
Preheat the oven to 400°F.
Remove lamb from plastic wrap, place the lamb on a wire rack over a baking sheet, place in the bottom 1/3 of the oven.
Roast until the internal temperature of the lamb is 135°F in the thickest part (use an oven-safe meat thermometer if possible), about 45 to 75 minutes. Remove from oven, allow to rest for 5 to 10 minutes (internal temp will rise to 140°F while it rests) before slicing into ½ inch strips.
In a small bowl, stir together all the mango salsa ingredients, set aside.
In a separate bowl stir together the harissa sauce ingredients, set aside.
Divide the lamb evenly between the four naan pieces. Top with desired amount of salsa, harissa sauce, and additional fetta and cilantro (if using). Serve immediately.