A sourdough starter is flour and water fermented by wild yeast and lactic acid bacteria. Feed it equal weights of flour and water every day until it doubles within a few hours of a feed. Mix the levain with flour, water and salt, then fold the dough every half hour for two hours. Let it rise slowly in the fridge overnight for a deeper, tangy flavour. Bake the loaf in a preheated cast-iron pot at 250 °C with the lid on for twenty minutes, then without the lid until the crust is deep brown.
