Make the rub: In a spice grinder or food processor, grind fennel and coriander, then combine with sumac.
Prep the lamb: Lay lamb flat and rub inside with 2 tablespoons of spice blend, plus salt and pepper. Fold lamb back up and rub exterior with remaining spice blend and more salt and pepper. Roll and truss the lamb into a bundle so that it cooks evenly.
Grill the lamb: Place coals on one side of grill and heat to about 350 degrees. Sear lamb fat-side-down directly over coals for 5 minutes. Flip and sear the other side for another 5 minutes. Remove lamb from direct heat and place on the other side of the grill. Let cook until internal temperature reaches 130 to 135 degrees, about an hour to an hour and 15 minutes. (You may need to add a few coals after about 30 minutes.) Let lamb rest for 25 to 35 minutes before slicing.