In a food processor puree the marinade ingredients. Pour the marinade into a large bowl. Add the lamb to the marinade, turn to coat, cover, and refrigerate for 4 to 6 hours.
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Preparing Boneless Leg of Lamb
Remove the lamb from the bowl and reserve the marinade. Pour the marinade into a small saucepan, bring to a boil, and boil for one full minute. Remove from the heat and reserve. Roll and tie the leg of lamb with butcher's twine.
Prepare the grill for rotisserie cooking over indirect medium heat. If your gas grill has one, turn on the infrared burner to low and set the outer burner control knobs to low heat. The temperature of the grill should be around 400°F (you may need to adjust the outer burners to medium heat).
Carefully slide one pronged fork into the spit, with the tines facing inward, about 10 inches from the end of the spit. Secure the fork but do not tighten at this time. Slide the spit through the center of the roast and gently push the roast onto the fork tines so that they are deep inside the roast. Add the other pronged fork to the spit with the tines facing inward and slide down until they are firmly imbedded into the roast. Secure the fork, but do not completely tighten at this time. Wearing barbecue mitts, place the pointed end of the spit into the rotisserie motor. If necessary, adjust the roast so that it is centered on the spit and tighten the fork into place. Turn on the motor to start the rotisserie.
Grill the roast until the internal temperature reaches 145ºF, about 1½ hours, basting the lamb with the reserved marinade every 20 minutes. To check the temperature, turn off the rotisserie motor and insert a thermometer into the center of the roast. Wearing barbecue mitts, carefully remove the spit from the grill. Gently loosen the forks and slide the roast off the spit. Transfer to a cutting board, tent with foil, and let rest for about 20 minutes before carving (the internal temperature of the roast will rise a few degrees during this time).
In a medium saucepan over medium heat on the stove, warm the beans until heated through. Add the tomatoes, olives, oil, parsley, and vinegar and mix well. Serve warm with the lamb.