In a large mixing bowl, whisk the starter into the water until mostly dissolved. Add the unbleached all-purpose flour, salt, and sugar. Stir until no dry spots remain and a shaggy dough forms.

Cover and let the dough rest for 30 minutes. After resting, begin a series of stretch and folds every 30 minutes for 2 hours. During the second fold, gently incorporate the cranberries and walnuts, ensuring they spread evenly without tearing the dough.

Let the dough ferment at room temperature for 4–6 hours, depending on the warmth of your kitchen. It should nearly double in size, look airy, and jiggle slightly when moved.

Turn the dough onto a lightly floured surface, shape into a round or oval loaf, and transfer to a floured banneton or lined bowl. Cover and refrigerate overnight (8–12 hours) for a slow, cold proof. This step develops flavor and improves the loaf’s structure.

Preheat your oven to 450°F (232°C) with a Dutch oven inside. Carefully transfer the shaped loaf to parchment paper, score the top, and place it in the Dutch oven. Cover and bake for 35 minutes at 450°F.

Reduce the oven to 400°F (204°C), remove the lid, and bake for an additional 15 minutes uncovered. The loaf should be golden brown with a crisp crust.

Transfer the bread to a wire rack and allow it to cool for at least 1 hour before slicing. Cooling ensures the crumb sets and locks in all those delicious cranberry and walnut flavors.