Feed your starter about 8 to 12 hours before preparing your dough.
In a bowl pour the sourdough (already activated) and the water. Mix.
Add the rye flour, unbleached flour and salt. Mix roughly to obtain a paste (no need to be homogeneous).
Film with plastic wrap. Leave to rest for 8 to 12 hours at room temperature.
Place the dough on a floured surface (it will be a little sticky, also flour your hands). Work it a little with the heel of your hand to obtain a homogeneous paste (lets say about 10 times), and add the walnuts.
Place parchment paper in the bottom of the dutch oven and place the loaf on top. Seal with plastic wrap.
Boil some water then place it in a bowl inside the oven off, on the lower rack. Place the dutch oven on the middle rack. Leave to rest for 2 hours.
Take the dutch oven and the bowl of water out of the oven, and preheat it to 230°C (450°F).
Score the dough with a blade or a sharp knife, put the lid on the dutch oven and place it in the oven for 30 min. Then remove the lid and cook for another 10 minutes.
Let cool before enjoying.