Place the lamb leg in a large baking dish. Pierce the lamb in about as many equally spaced slits as you have slices of garlic. (No need to count them. You're cooking.) Slide the garlic into the pockets.
PREP THE MARINADE Combine the grated onion, cucumber juice, parsley, lemon juice, oil, oregano, and thyme in a medium bowl.
Pour the onion mixture over the lamb leg and marinate in the refrigerator for at least 4 hours, preferably longer--as long as 24 hours.
GET READY TO GRILL... If you have a rotisserie-type contraption for your grill, fantastic. If not, don't sweat it. Ready your grill for low-slow cooking over indirect heat. Charcoal is ideal. Gas, in this instance, is perfectly acceptable.
Remove the lamb from the marinade and discard the marinade. Rub with 1 teaspoon
of the salt. Place the lamb leg on the grill over indirect, medium-low heat. Cover and cook 1 1/2 hours, turning every 15 minutes or so.
Fan the onion slices on the unheated side of the grill over indirect heat. Place the lamb on top of the onions, allowing them to slowly cook--all the while being basted by the lamb.
Cook the lamb, covered, another hour, turning every 15 minutes. Remember, spit-style Greek lamb is typically cooked well done and beyond, until the leg is nearly fork-tender.
Remove the lamb and the onions from the grill, allowing the lamb to rest for 10 minutes before carving.
Remove the netting and carve the lamb into thin slices, reserving at least 1/4 cup of the juices for stirring into the yogurt sauce.
MAKE THE SAUCE AND SERVE
To prepare the sauce, combine the Greek yogurt, the red wine vinegar, 1/8 teaspoon salt, the pepper, and the reserved lamb juices in a small bowl.
Fan the tomatoes on a large serving platter followed by the onions, followed by the sliced lamb, followed by the yogurt sauce.