To prepare an arni souvlaki recipe, first you need to prepare the lamb. First, you'll need a lamb tenderloin that's at least 3 to 4 inches long. Then, cut your lamb into 12 inch thick slices. Next, place the cut lamb in the oven and cook it for 1 to 2 hours, or until the meat reaches an internal temperature of 145 degrees F (60 degrees C).