To make a sourdough starter: whisk 50g of the strong white flour and 50g of the strong wholemeal flour with 100ml slightly warm water until smooth. Transfer to a large jar or plastic container. Leave the lid ajar for 1 hr or so in a warm place, then seal and set aside for 24 hours. For the next six days, you will need to ‘feed’ it. Each day, tip away half the original starter, add an extra 25g of each flour and 50ml slightly warm water, and stir well. After a few days you should start to see bubbles on the surface, and it will smell yeasty. On day seven, the starter should be bubbly and smell much sweeter. It is now ready to be used.
To make the sourdough bread: tip both the flours, 325ml warm water, the salt and the starter into a bowl, or a mixer fitted with a dough hook. Stir with a wooden spoon, or on a slow setting in the mixer until combined – add extra flour if it’s too sticky or a little warm water if it’s dry. Tip onto a lightly floured surface and knead for 10 mins until soft and elastic – you should be able to stretch it without it tearing. If you‘re using a mixer, turn up the speed a little and mix for 5 mins.
Place the dough in a floured bowl and cover with cling film. Leave in a warm place to rise for 3 hrs. You may not see much movement, as sourdough takes much longer to rise.
Line a medium bowl with a clean tea towel and flour it really well or flour a proving basket. Tip the dough back onto your work surface and knead briefly to knock out any air bubbles. Shape the dough into a smooth ball and dust it with flour.
Place the dough, seam-side up, in the bowl or proving basket, and leave at room temperature for 3 hrs, or in the fridge overnight, until risen by about a quarter.
Place a large baking tray in the oven and heat to 230C/210C fan/gas 8. Fill a small roasting tin with water and place in the bottom of the oven to create steam. Remove the tray from the oven, sprinkle with flour, then tip the dough onto it.
Slash the top a few times with a sharp knife to make a pattern, then bake for 35-40 mins until golden brown. It should sound hollow when tapped on the bottom. Leave to cool on a wire rack before slicing.