1. Dilute the sourdough starter with the water, use warmer water if the starter has been kept in the fridge.
2. Place the seeds into a large bowl, add the flours and salt. Mix the diluted starter into the flours until it’s evenly distributed, using a spatula or a dough whisk.
3. Leave for 30 minutes and then stretch and fold the dough a few times with oiled hands every 30 minutes for 2 hours. Refrigerate for 12 hours.
4. The next day bring the dough to room temperature, it will take 1-2 hours, depending on how warm your kitchen is. Turn it out onto a floured worktop, pat it gently into a flat round and pre-shape a loaf by folding edges into the middle. Turn it seam side down and leave to rest for 25 minutes, covered with a cloth.
5. To shape a round loaf, turn the dough seam side up and flatten to a disc. Lifting the edge close to you, roll it away until you get a fat sausage. Flip it seam side up and roll in the opposite direction into an even fatter sausage. Flip it again seam side down, cup its far side with your both hands and gently drag it towards you. You’ll feel the top smoothing and tightening. Turn it 90 degrees and drag it towards you again. If the dough sticks, flour your hands lightly.
6. Place it in a well-floured banneton or a bowl lined with a floured cloth, seam side up. Cover or place in an inflated plastic bag (just blow into it and tie the ends) for 40 minutes.
7. Preheat the oven to 220C/425F/gas 7 with a medium sized (23cm) cast iron dish inside.
8. When the time is up the loaf won’t have risen much but it should spring in the oven. Turn it swiftly out into the cast iron dish (better to remove it from the oven for that bit), slash the top with a serrated knife or a baker’s lame, put the lid on and transfer into the oven.
9. Bake it for 20 minutes with the lid on and 20 with the lid off. Shake it out of the dish and cool on a wire rack.