For the starter, combine 200g of the flour with 200ml of lukewarm water in a non-metallic container – a glass jar is perfect. Leave it somewhere warm uncovered overnight.
The following day, feed it by discarding half and adding a further 100g of flour and 100ml of lukewarm water. Repeat this feeding process each day until you see bubbles throughout the mixture (this is where a glass jar comes in handy). It will take a few days, possibly more, for the mixture to pick up the natural airborne yeasts and really start living. Don’t lose hope, it will happen!
Removing 100g of the starter to make your loaf, chill the rest, covered but with a hole for it to breathe. You’ll need to feed this once a week – bring it up to room temperature first, allow it to bubble up, then recede (somewhere warm) for 30 minutes to 1 hour, then return it to the fridge within an hour or so. A liquid layer may develop on top, called hooch; stir it in if you like, or pour it off.
To make the sponge, bring the 100g of starter up to room temperature. Combine the 300g flour and ½ teaspoon of sea salt in a bowl, then add 300ml of warm water and the starter. Cover with a towel and leave it to rise somewhere warm for 3 to 4 hours, or until doubled in size.
In the meantime, top up the starter with 50g of flour and 50ml of lukewarm water (the weight you removed), allow it to bubble up and down as in step 3, then return it to the fridge. Either use your sponge straight away or chill it, covered, overnight.
Now to make the sourdough bread. Bring the sponge up to room temperature (if needs be), combine it with the flour and 2 teaspoons of salt, then turn it out onto a floured surface. Knead for 10 minutes, or until smooth and elastic. Put the dough in a lightly oiled bowl, cover and leave to rise in a warm place for 2 to 3 hours, or until doubled in size.
Tip the dough onto a floured surface, knock out the air, then shape it into a round loaf on an oiled baking sheet. Cover and prove for about 1 hour in a warm place, or until it doesn’t spring back when prodded.
Preheat the oven to 220°C/425°F/gas 7.
When the dough is ready, place a tray of water in the base of the oven (this will help it develop a good crust) and score the top of the loaf with a sharp knife. Bake for 30 to 40 minutes until golden and sounds hollow when tapped. Cool on a wire rack.
* You can use the yeast to accelerate your starter. Add it with the flour at the beginning and it will bubble up within a few hours – no need to leave it overnight. Feed it once, let it bubble up and descend again for around 1 to 2 hours, then continue as in step 3.