I have one! This makes 2 loaves which you might as well do if you are doing all this work.
Night before: Make levain by mixing 100g flour, 100g water and 1 tablespoon Sultana. Mix well, cover with clingfilm and leave in warm place overnight.
Next morning:
Squish together 700g plain white flour, 300g spelt or granary or just more white flour, and 700mls warm water. Leave to develop gluten 2-4 hours.
Squish levain through, wait 1 hour.
Add 20g salt. Now - every 30 mins, for anywhere between the next 2-6 hours, do a stretch and fold in the bowl and cover back up again.
When the dough is poofy and ready to go, divide into two balls, shape, dust really well, put into two bowls lined with tea towels (or bannetons) and leave in the fridge uncovered overnight/12 hours or so. Here's a video to show that stage: bit.ly/3dRsLJM
Next morning:
Preheat oven to 240c with a cast iron pot inside with a lid. When the oven is to temp and the pot has been sitting in there for at least 15 minutes and blazing hot, drop dough straight into pot from the bowls and slash the top with a razor or snip in a line with scissors. Clap the lid back on and bake 45 mins. Take the lid off for the last 5 mins if you want it a bit darker.