Combine 125g rye sourdough starter, 150g strong white flour, 50g dark rye flour and 200g water. Mix well, cover and leave to rest at room temperature for 16 to 24 hours.
Combine 400g of the sourdough (the rest of it goes back into the fridge for your next bake). Add 450g strong white flour, 100g dark rye flour, 150g chestnut flour, 10g salt and 400g water to form the dough.
Knead for 10 minutes. The dough will be a bit sticky but don't be tempted to add more flour.
Place the dough into a bowl, cover with a lid and rest at room temperature for 1 - 2 hours.
Give the dough another quick knead, then place in a lightly buttered large baking tin (mine is 31 cm x 12.2 cm x 8.6 cm).
Cover with a polythene bag and rest at room temperature for several hours until fully proved and significantly increased in size.
Bake in a preheated oven at 220°C for 10 minutes, then at 200°C for a further 45 minutes.
Cool on a wire rack.