2-1/2 lb boneless butterflied leg of lamb (a bit larger or smaller won't make a difference)
1/2 cup plain yogurt (I use nonfat)
Zest of 1 lemon
4 cloves garlic, peeled and sliced
1/4 tsp chopped fresh rosemary or oregano
1/2 tsp kosher salt
1/4 tsp fresh black pepper
3 Tbsp extra-virgin olive oil (try to find a fruity oil)
Remove the lamb from the refrigerator. Dry with a paper towel and set aside while you make the marinade.
In a large bowl, combine remaining ingredients and mix well. Add the lamb to the bowl, and massage to coat completely with the marinade. Transfer the meat and all of the marinade to a large ziploc plastic bag. Squeeze the air out of the bag, seal, and flatten the lamb so the marinade coats the entire piece of meat. Refrigerate for at least 12 hours, or up to 2 days (the longer, the better).
To cook, heat a grill to 500°F. While the grill is heating, take the lamb out of the refrigerator and let it come to room temperature. Wipe off all of the marinade, and discard. If necessary to create a uniform thickness (it usually is not), run some long metal skewers through the meat to hold it flat on the grill. When the grill is hot, cook the lamb for 5 minutes on the first side. Turn, and cook for 10 minutes on the second side. Then, remove the lamb from the grill and take its temperature with an instant-read thermometer inserted into the thickest part; if the temperature registers 125F (for rare lamb), remove the meat from the grill, and if not, put the meat back on the grill for a minute or two.
Remove from the heat and let the meat sit for at least 10 minutes before slicing. The thicker parts will be rare; the thinner parts will be more well done.