Butter an ovenproof dish and set aside.
In a saucepan, bring about 600 ml of water to a boil and add the salt. Whisk the polenta with the remaining water and pour into the boiling water. Reduce heat and stir with a whisk until the mixture thickens.
Cook on low heat for another 10 minutes, stirring constantly, until the mixture becomes very thick.
Now spread the polenta mixture into the buttered pan and bake in the oven preheated to 180 °C for 15 minutes.
Remove the polenta, turn it out and cut into slices.