Make the marinade: In a large zip-top bag, combine 1/3 cup olive oil, 4 cloves smashed garlic, jalapeño, one chopped shallot and salt and pepper. Add the lamb and marinate for four hours at room temperature, or refrigerated overnight. If refrigerated, let come to room temperature before grilling.
Once the lamb has marinated, bring your grill to very high heat. Charcoal grill is best here, but any hot grill will do. Place lamb on the grill, making sure the meat isn’t bunched up in places and is completely flat. Cook for about 8 minutes, at which point there should be a lot of charring, then flip the meat over and cook for another 8 minutes. If the lamb is particularly thin (less than 2-inches thick) you may only need 5 minutes on each side. Insert a meat thermometer into the thickest part of the lamb to make sure the temperature has reached 145 degrees fahrenheit. Transfer lamb from the grill to a cutting board, cover with foil and let rest for ten minutes. When the lamb is nearly finished resting, cut it into strips against the grain.
While the lamb rests, make the salsa: In a medium mixing bowl, combine the remaining 1/3 cup olive oil, minced garlic, remaining chopped shallot, red wine vinegar, cilantro, parsley, oregano, red pepper flakes, squeeze of lime, salt and pepper and whisk together.
To serve, warm your tortillas on the grill and assemble tacos by filling with lamb, salsa, cotija, and radish.