1 (5 1/2-lb) piece boneless butterflied leg of lamb
1-1/2 Tbsp Kosher Salt
For Rosemary Salt
1 tsp finely chopped fresh rosemary
1 Tbsp Coarse Sea Salt
Stir together yogurt, garlic, rosemary, and pepper in a 13- by 9-inch glass baking dish or in a big (2-gallon) resealable plastic bag. Add lamb to marinade, turning to coat completely, and marinate, covered, in the refrigerator, for five hours. Turn lamb and marinade over once or twice during marination time.
Take lamb from refrigerator and bring to room temperature, about 1 hour.
Remove lamb from marinade, discarding marinade, and put leg of lamb on a work surface. Run skewers horizontally through meat, about 1 1/2 inches apart, first lengthwise (4 or 5 skewers), then crosswise (4 or 5 more) to form a grid. (Skewering in this fashion makes meat easier to move and turn over.) Sprinkle leg of lamb with kosher salt.
Prepare grill for cooking.
If using a charcoal grill, open vents on bottom of grill, then light charcoal. Charcoal fire is medium-hot when you can hold your hand 5 inches above rack for 3 to 4 seconds.
If using a gas grill, preheat burners on high, covered, 10 minutes, then reduce to moderate.
Grill lamb, covered only if using a gas grill, on lightly oiled grill rack, turning occasionally, until thermometer inserted diagonally into thickest part of meat registers 125°F for medium-rare (thinner parts will register higher). This should take 25 to 30 minutes if using charcoal, or 20 to 25 minutes if using gas.
Transfer lamb to a cutting board, and let stand 10 minutes before thinly slicing. The internal temperature of the leg of lamb will rise a little during this time, and juices will redistribute throughout the meat.
Make rosemary salt:
Stir together rosemary and sea salt.
Serve the leg of lamb garnished with the rosemary salt.