Rosemary Walnut Buttermilk Country Sourdough

Steps

1. Day 1, about 8 a.m.: feed your sourdough starter at a room temperature of about 75°F (24°C). It should peak, doubled and domed with bubbles, in 4 to 6 hours.
2. While the starter rises, toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes until fragrant, then cool and chop. Soften the butter and set aside about two thirds for the dough and the remaining third for brushing.
3. When the starter peaks, whisk the water and buttermilk in a large bowl, then stir in the sourdough starter until mostly dissolved.
4. Add the all-purpose flour and the whole wheat flour and mix until no dry bits remain. Cover and rest 30 minutes.
5. Sprinkle the salt and rosemary over the dough and add the two thirds of the butter. Squeeze and fold until fully worked in, about 3 minutes. The dough is about 71% hydration and 2% salt, counting the starter's water and the buttermilk as liquid and the starter's share of grain toward the total.
6. Bulk rise covered at about 75°F (24°C) for 4 to 5 hours. In the first 2 hours do 4 sets of stretch-and-folds with wet hands, 30 minutes apart; spread the walnuts over the dough during the first set and fold them in. Bulk is done when the dough has grown about 50%, feels airy and jiggles.
7. Turn the dough onto a clean counter, pre-shape into a loose round with wet hands and rest 20 minutes. Shape into a tight boule and place seam side up in a bowl lined with parchment.
8. Cover and cold proof in the fridge for 12 to 14 hours overnight.
9. Day 2: line a baking sheet with parchment and preheat the oven to 425°F (218°C) for 45 minutes. Turn the cold dough onto the parchment and score the top with a sharp blade.
10. Bake 38 to 42 minutes until deep golden and the center reads 200°F (93°C).
11. Brush the hot loaf with the remaining third of the butter, then cover loosely with a clean towel; this keeps the crust soft.
12. Cool on a rack for at least 2 hours before slicing into 12 slices.