02Put the lamb on a large platter, and trim any excess fat. Cut the lamb in 1/2 lengthwise so you have 2 large pieces. Make slits all over the meat with a sharp knife, and push the garlic slivers inside. Rub 1/2 of the garlic-oregano paste all over the lamb and season the meat with salt and pepper. Roll up each piece of lamb lengthwise and tie with butcher's twine to hold the roasts together. Put the lamb on a large platter and slather with the remaining garlic-oregano paste. Marinate, covered, in the refrigerator for at least 1 hour or up to overnight.03Preheat the oven to 375 degrees F.04Wipe off the excess paste from the lamb so it won't burn in the oven. Place a large roasting pan over 2 burners on medium-high flame and coat with a fair amount of olive oil. Put the lamb roasts in the roasting pan and sear to form a nice brown crust on all sides. Transfer the pan (and lamb) to the oven and roast for 1 to 1 1/2 hours; it should be pink in the center and the internal temperature reads 130 degrees F. Allow the lamb to stand 10 minutes to let the juices settle before and slicing.05Cut off the butcher's twine and thinly slice the lamb. Place the meat in the center of the pita breads. Top with lettuce, chopped tomatoes, and tzatziki.06Put all the ingredients in a mixing bowl and combine with a fork. Refrigerate for at least 1 hour to allow the flavors to marry.