It's steamed in a couscoussier, the same pot Tunisians use for couscous, with rice taking the place of semolina.
Rice is mixed with chopped spinach, parsley, dill, onion, chickpeas and pieces of lamb, chicken or fish, seasoned with tomato paste and spices, then steamed until soft and fragrant.
Why it's made this way: Djerba's Berber, Jewish, Arab and African influences produced a distinct island cuisine built on rice rather than couscous, and riz djerbien is considered the dish that best captures that blend.