Feed your starter with flour and water to activate the sourdough. Leave it to ferment until bubbly and doubled in volume.
Work all the dough ingredients for 10 minutes into a soft and slightly sticky mixture.
Cover and prove the dough for 4 hours at room temperature, and then in the fridge for another 4 hours, or overnight.
For the oil spread, whisk oil, flour, and a pinch of salt into a smooth paste. Then, refrigerate it for 20 minutes.
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{{/ items}} {{^ items}} {{/ items}}Roll the dough into a square and spread the oil paste over the centre. Then, pack it up, folding the dough edges over the spread, and roll it out into a long rectangle.
To laminate the pastry, fold the bottom third of the dough rectangle over the centre and then the top third, like a letter.
Then, roll it out and transfer it, seam side down, onto a tray. Prove for 1-2 hours in the fridge; repeat the folding step twice.
Thinly roll out the dough, cut it out into triangles, and roll each of them into a croissant.
Transfer the croissants to a tray and leave to prove covered for 2 hours.
Bake the croissants for 18 minutes at 190°C (390°F) until golden.