Peel the cooked potato and grate finely.
Put the water and the rest of the ingredients in a mixing bowl.
Knead until a smooth dough is formed.
Cover and leave to rise (for 6 hours) or overnight, at room temperature.
Transfer the dough to a floured work surface and shape into a ball.
Flour a fermenting basket well and place the dough inside. (I don't have a fermenting basket, so I used a bowl with a tea towel and flour it well).
Leave to rise for about another hour at room temperature.
Preheat oven to 220° C.
Turn the bread out onto a tray with baking paper.
Place in the oven and bake for 10 minutes with plenty of steam.
Then turn down to 180° C and bake with steam for another 40 minutes.
Leave to cool on a rack.