For the lamb: Let the lamb come to room temperature, about 30 minutes before you are ready to cook it. Season all sides thoroughly with salt and pepper.
Preheat the oven to 325 degrees F.
Heat a large, heavy-bottomed skillet with an ovenproof handle, over medium-high heat. Place the lamb fat-side down, and cook until it is browned, about 3 minutes per side. Place the pan with the lamb in the oven and roast for about 15 to 20 minutes, or until the internal temperature reaches 130 degrees F for medium-rare or 140 degrees F for medium. Remove the lamb from the oven, cover it with foil, and let it rest for about 10 minutes before slicing. Serve with Chimichurri sauce, recipe to follow.
For the chimichurri sauce: Add the garlic to the bowl of a food processor or blender and pulse to get it chopped up. Place the parsley, chives, cilantro, mint, red pepper flakes, vinegar, and lime juice into the food processor and pulse to begin to combine. With the machine running slowly drizzle in the olive oil until well incorporated. Serve the sauce with the lamb.