In a large bowl, mix together the bread flour, whole wheat flour, and water. Cover the bowl with plastic wrap or a clean damp kitchen towel and allow the dough to rest for 60 minutes.
Mix in the starter and leave the dough to rest for 30 minutes.
Add the salt, mix, and leave the dough to rest for another 30 minutes.
Perform the first stretch and fold, then leave the dough to rest for 45 minutes.
Perform the second stretch and fold, then leave the dough to rest for 45 minutes.
Perform your third stretch and fold, then leave the dough to rest for 45 minutes.
Perform the fourth and final stretch and fold. Allow the dough to rest for four hours.
On a well-floured work surface, divide the dough in half and shape each half into a round. Transfer each round seam-side up into a proofing basket, then refrigerate the dough for 12-24 hours.
Preheat the oven to 450°F with two Dutch ovens inside. If your oven can only fit one, keep the other loaf refrigerated while you bake the first one. Remove the loaves from the fridge, flip onto parchment paper, and score each one.
Immediately transfer the loaves to the preheated Dutch ovens, cover, and bake for 25 minutes.
Remove the lids and bake the loaves for an additional 20 minutes, or until they are golden brown. Allow the loaves to cool for at least 10 minutes before cutting in. Enjoy!