Put the sourdough starter in the mixing bowl and add the 50ml water. Use a dough whisk (or fork) to mix the starter into the water. Add the white flour and mix with the whisk. Cover with a cloth and leave for 4 hours.

Add 250ml water and mix the leaven with the spatula. Add the white and khorasan flour and then mix with the dough hook for 5 minutes, first slowly for 2 minutes and then at medium speed for 3 minutes.

Cover the bowl with cloth and leave to rest for 2 hours.

Sprinkle the salt on the surface of the dough and mix with dough hook for 8 minutes.

Transfer the dough to an oiled bowl or round plastic tub with a lid. The bowl should be large enough to allow expandion of up to 2 1/2 times the volume of the original dough. Cover the bowl with cloth and allow to rise for 1 or 2 hours.

Prepare a large banneton/proofing basket coating well with rye flour.

Take the dough from the fridge and empty onto a floured work surface (rye flour) forming into a ball.

Starting at the top, stretch the dough from underneath and pull on to the top of the ball. Then do the same from the side nearest to you and then stretch the dough in the same way from left and right sides.

Put the dough into the banneton keeping the top as was on your work surface still at the top. Cover with the linen cover and leave to rise for 3 hours.