On day 1, prepare the sourdough by combining the sourdough starter, wholemeal rye flour and water in a medium bowl. Mix well, then cover and leave to rest for 16 -24 hours at room temperature.
On day 2, once the sourdough starter is ready, combine 260g of the refreshed sourdough (the rest goes back into the fridge for your next bake), the strong white wheat flour and wholemeal rye flour, the buttermilk (ideally at room temperature), the water and salt in a large bowl.
Combine to form a dough, then turn it out onto your work surface.
Knead for 10 minutes.
Place the dough back into the bowl, cover and leave to rest for 30 minutes.
Turn out the dough and knead for 1 minute.
Place back into the bowl again, cover and leave to rest for a further 30 minutes.
Turn out the dough and shape into a boule.
Bathe the boule in a flour bath so it's no longer sticky on the outside, then place seam-side up into a pre-floured proofing basket. Proof for several hours until risen - it takes about 3 hours in my kitchen but as always the proofing time depends very much on the temperature in your room so it might take a little shorter or longer - then preheat the oven to 200°C, and - if using - your baking dome at the same time.
Turn out the fully proofed loaf onto the preheated baking dome plate (if using) or a baking tray lined with baking paper.
Make several decorative slashes with a scoring knife. Put the cover on the baking dome plate (if using).
Bake for 15 mins at 200°C, then turn the temperature down to 180°C and bake for a further 40 mins. Take off the baking dome cover for the last 10 minutes to finish the crust.
Cool on a wire rack.