BRING THE WATER TO A BOIL, add the salt, then whisk in the polenta, pouring it in a slow, steady stream so that lumps don't form. Lower the heat, and cook about 30 minutes, while stirring. Or cook the polenta in a double boiler over simmering water for 45 minutes, giving it a stir every 15 minutes or so. If you wish to serve the polenta soft, keep it over the heat in a double boiler until needed. To make firm polenta, pour it into a pan and set it aside; after a while it will cool and firm up. Turn it out of its pan and cut it into 1 cm slices. To broil or grill the polenta, brush both sides of each slice with olive oil and cook until lightly browned. The slices can also be fried in butter or olive oil, either until lightly colored or until crisp, and they can be used as layers in a casserole.