Place the sheet of vegetables directly on the grill grates underneath the lamb. Turn on the rotisserie motor, cover the grill, and cook, stirring the vegetables every 30 minutes, until an instant-read thermometer inserted into the center of the lamb registers 135°F to 140°F, about 1 to 1 1/2 hours.Turn off the rotisserie motor, rotisserie burner, and grill burners. Using potholders or kitchen towels, carefully remove the spit to a cutting board and loosen the forks.