1. Using the tip of a sharp knife, cut evenly spaced small slits about ½ to 1 inch deep all over the lamb. Insert pieces of garlic clove in the slits. You should have 20 or more slits.
2. Place the salt, pepper, dried thyme, and dried oregano in a small bowl and stir to combine.
3. Tear off a piece of aluminum foil large enough to completely wrap the leg of lamb and place it on a work surface. Arrange large piece of plastic wrap on top of it. Place the garlic-studded leg of lamb on top of the plastic wrap. Rub the herb mixture all over the leg of lamb. Arrange the lemon slices and stalks of rosemary all over the lamb, placing some underneath the leg and some on top. Wrap the lamb tightly with the plastic wrap, then with the aluminum foil. Refrigerate the lamb until ready to cook, at least 8 hours, preferably overnight.
4. When ready to cook, unwrap the lamb and remove the lemon slices and rosemary sprigs and discard them. Rub the lamb lightly with olive oil.
S. Preheat the grill to medium-high heat.
6. Skewer the lamb on the rotisserie spit, if using, and grill according to the manufacturer’s instructions until cooked to taste, 1¾ to 2 hours. When done to medium-rare the internal temperature of the lamb will read 125°F on an instant-read meat thermometer (take care not to touch the bone when inserting the thermometer). When done to medium-well the temperature will register 140°F. You can also grill the leg of lamb on a medium-high grill over indirect heat, turning it often; it will be done after 1¾ to 2 hours.
7. Remove the lamb from the rotisserie spit, if using, and place it on a platter to rest for about 15 minutes before carving. Or, let the lamb cool completely, wrap it in aluminum foil, and refrigerate it. Rewarm the lamb by opening up the foil on top and placing the lamb in a 350°F oven for about 30 minutes before serving.
8. Carve the lamb into slices. Garnish the platter with sprigs of fresh thyme, oregano, rosemary, if desired, and lemon wedges. Serve the Greek Chimichurri Sauce on the side.
For the sauce:
Place the onion, olives, mint, parsley, oregano, garlic, lemon juice, and salt in a food processor fitted with a steel blade. Process until finely chopped and well blended, about 1 minute.Taste for seasoning, adding more lemon juice as needed. Spoon the chimichurri sauce into a small glass bowl, cover it with plastic wrap, and refrigerate until serving time.