For toasted cumin: Heat a small skillet over medium heat. Add cumin seeds and cook; stir/shake pan for several minutes, until seeds are toasted and fragrant. Transfer to a mortar or spice mill and grind to a coarse texture. Transfer to a medium-size bowl.
Add the herbs,garlic, lemon zest, salt and pepper and mix well. Drizzle in as much oil as needed to make a paste. Rub it all over the lamb.
Wrap the lamb in plastic or place it in a large plastic bag or bowl and let marinate at least 2 hours or refrigerate overnight. Turn lamb occasionally if using a bowl.
Let the lamb come to room temperature if refrigerated. Heat a grill to very hot.
Do not remove marinade from lamb. Grill the lamb 12 to 15 minutes per side. For medium-rare you want an internal temperature of 125 degrees. Since the shape of the meat is uneven, you may want to remove some thinner sections before the rest is done.
Remove the lamb to a cutting board, tent with foil and let rest 15 minutes before slicing.