Place the flour (345g white and 55g rye+spelt) and water (280g) in a large bowl and mix it using a wooden spoon or a spatula until the water is fully incorporated into the flour. Cover with cling film and leave 1 hour for autolyse.
Add the sourdough starter and gently knead it by hand into the four & water mix. Cover with cling film and allow it to rest for 30 minutes.
Add the salt and knead by hand or using a stand mixer until the gluten is fully developed.(see notes)
Leave the dough in the mixing bowl, cover with clig film.
Perform 3 sets of stretch and fold, straight into the bowl, at 45 minutes intervals. This will help strenghten the gluten bonds.
After the third set of stretch and fold, leave the dough to proof at room temperature for at least 3 hours or until it has at least doubled in volume.
Take the dough out on a lightly floured surface. Degase, preshape and allow to rest for 10 minutes.
Prepare the banneton by lining it with flour. If you're using a proofing basket/sieve/bowl you might want to line it with a well floured kitchen towel.
Shape the bread and place it into the banneton/proofing basket. Cover with a clean kitchen towel and place it in the fridge to proof overnight.