Lightly oil a 9-inch round glass baking dish. In a medium saucepan, bring 3 cups of water to a boil. Add 1 1/2 teaspoons of salt and slowly whisk in the polenta. Cook over low heat, whisking constantly, until the polenta is thick, about 5 minutes. Pour the hot polenta into the baking dish and cover the surface directly with plastic wrap. Let stand until firm, about 25 minutes.