Prepare the sourdough base, also called the 'levain'. Mix the active sourdough starter with the flours and water listed in the levain ingredients to make a relatively thick paste. Cover the bowl with cling wrap/film and leave it for 20-24 hours at a cool room temperature.
Evening day 2:
The levain might not look like much from above, but if you have a clear bowl you should see bubbles from the side. Add all of the other ingredients to the levain and mix to combine.
Lightly flour a work surface, tip the dough out and knead the dough for around 3-5 minutes, adding a little extra flour if needed. It will be soft and a little sticky, but shouldn't be so sticky that you can't handle it.
Leave the dough as a ball on the counter and cover with an up-tuned bowl for an hour (if the bowl is only slightly larger than the dough, lightly oil it so it doesn't stick).
Prepare a round banneton by dusting with flour, or else either line a relatively large bowl with a clean towel and dust with flour, or simply dust a large bowl with flour, turning so it covers up the sides. Fold in the sides of the dough tightly to form a round ball and transfer to the prepared banneton/bowl, join side up. Cover and place in the fridge to rise slowly overnight.
Morning day 3:
Preheat the oven to 450F/230C. Cut a rectangle of parchment paper large enough to cover the bottom of a Dutch oven ad come up the sides slightly - I typically test it as I'm about to cut. Then put the Dutch oven in the oven to warm as the oven heats up.
Take the bread dough from the fridge and ease it away from the sides gently, if needed. Turn the dough upside down, onto the middle of the piece of parchment (so the join side is now down). If the top of the dough doesn't look a little floured, dust it with a bit more flour.
Once the oven comes to temperature, slash the top of the dough with a lame or sharp knife, and carefully use the sides of the parchment to lower it in to the warmed Dutch oven. Bake the loaf for 20 minutes with the lid on, then remove the lid and cook for a further 20-25 minutes, until it has an even brown color and is hollow sounding when tapped. Allow the bread to cool at least 20-30 minutes on a cooling rack before slicing.
If you are concerned that your levain doesn't look that active after the day's rise, you can add ¼tsp instant dried yeast when you add the other ingredients the second day.