Add the water and starter to a large mixing bowl and whisk to combine.
Add the spelt flour, rye flour, and salt to the bowl, and use a wooden spoon or spatula to mix until a shaggy dough forms. Finish mixing with your hands to fully incorporate the flour. The dough will feel quite hard and dense, this is normal.
Cover the bowl and set aside to rest for 30 minutes.
Once the dough has rested, begin your stretches and folds. Do three rounds of stretches and folds over the course of an hour, once every 20 minutes.
Form the dough into a ball or boule by placing it onto a clean surface and using your hands to rotate until surface tension forms.
Line a banneton or round bowl with a tea towel and sprinkle with flour. Place the dough upside-down into the prepared basket.
Cover and set aside to rise at room temperature for two hours. The dough should visibly rise during this time.
Place the dough into the refrigerator overnight, or for at least eight hours. Cover with a plate to prevent drying.
Place a heat-safe dutch oven into the centre rack of your oven and preheat the oven to 250°C (480°F).
Turn the dough out onto a piece of parchment paper and score with a sharp knife (scoring optional).
Carefully remove the dutch oven from the oven, remove the lid, and place your loaf into it, using the parchment paper as handles to lift the bread.
Place the bread into the oven and reduce the temperature to 230°C (450°F).
Bake for 20 minutes with the lid on, then remove the lid and bake for another 15-20 minutes, or until browned to your desired degree.
Remove from the oven and cool in the pot for ten minutes before carefully removing the bread and cooling fully on a wire rack. It must be completely cool before slicing. For this loaf, I recommend cooling for a minimum of eight hours.
Store the bread in the pot you've baked it in, or freezing individual slices and toasting to thaw.