Dissolve the dry yeast and salt in lukewarm water.
Mix flour and water with a wooden spoon in a bowl until a lump of dough is formed.
Cover the bowl with cling film and leave to rise at room temperature for 18 hours.
Preheat the oven to 230°C top/bottom heat and heat a cast-iron roaster with lid for 30 minutes in the oven
Take the roaster out of the oven, put a baking paper into the roaster, sprinkle about 1 tbsp. flour on the bottom and pour the relatively liquid dough on the baking paper. Put the lid on and bake for 40 minutes. Then remove the lid and bake for another 25 minutes.