Put the flours, salt, mashed potato and sourdough starter in a large bowl.

Stir in the potato cooking water to make a wet dough, mixing very well to ensure everything is combined.

Cover the bowl and leave overnight (12-16 hours).

The dough should be well risen and very bubbly. Leave a little longer if necessary.

Scrape the dough out of its bowl and onto a floured worktop.

If making 2 small instead of 1 large loaf, divide the dough in half.

Fold the dough several times to shape it to the proving basket(s) or tea towel-lined bowl(s).

Cover and leave on the worktop to rest for 10 min.

After 10 min, dust the proving basket(s) or tea towel(s) with fine polenta, cornmeal or flour and put the dough inside.

Cover and leave for 1 hour - 1.5 hours until the depression left by a finger pressed into the dough slowly rather than quickly springs back.

In the meantime: preheat the oven to its highest setting and put inside one or two cast iron pots or Dutch oven(s) including lids.

When ready to bake, remove the pot(s) from the oven and carefully flip the dough from the proving basket(s) or bowl(s) into the pot(s).

Slash the top(s), replace the lid(s) and place in the oven.

Immediately turn the heat down to 240C/220C Fan/Gas 9.

Bake for 30 minutes without removing the lid(s).

After 30 minutes, reduce the heat to 220C /200 Fan /Gas 7.

Remove the pot(s) from the oven, take the bread(s) out, then return to sit directly on the oven shelf. Bake for a further 10-15 minutes until the bread is cooked through (should sound hollow when tapped underneath).

If necessary, reduce the oven temperature to stop the crust over browning before the bread is finished baking.

Place on a wire rack to cool before eating.

Bread should keep at least 3 days but can be successfully frozen.

Tip: slice before freezing so you can just take out as much as you need.