Prepare the sourdough by combining the sourdough ingredients in a bowl. Cover and keep at room temperature for 16 - 24 hours.
At the same time, prepare the oat soaker by dry-roasting the rolled oats in a big frying pan. Transfer to a bowl, pour over the hot water, cover and also keep at room temperature for 16 - 24 hours.
The day after, prepare the main dough by combining the sourdough, oat soaker, wheat flour, rye flour, water, salt and yeast in a large bowl.
Knead for a good few minutes until you have a homogeneous and even dough.
Place the dough back into the bowl, cover and rest at room temperature for about 2 hours.
Give the dough another quick knead, then place in a large lightly-oiled Pullman baking tin (33 x 10 cm).
Distribute the dough evenly.
Cover with the Pullman tin lid and leave for several hours at room temperature to proof. In my relatively cold Edinburgh flat, it took a good seven hours for the bread to be fully proved. It should roughly double in size.
Bake in a preheated oven, starting at 250°C for the first 10 minutes, then turning the temperature down to 200°C for another hour.
Cool on a wire rack.