Soft Cheddar Walnut Country Sourdough Boule

Steps

1. The night before mixing (about 8 pm), feed your starter equal weights of all-purpose flour and water from your own stock, separate from this recipe's ingredients, and leave it at about 75°F (24°C); it peaks, bubbly and doubled, in 8 to 12 hours.
2. Meanwhile toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes, cool and chop them, then cube the cheddar and keep it chilled.
3. In the morning, set aside about 10 g of the all-purpose flour for dusting. Whisk the water, milk and sourdough starter in a large bowl until milky.
4. Add the rest of the all-purpose flour and all of the whole wheat flour and mix until nothing dry remains; cover and rest 45 minutes.
5. Sprinkle the salt and drizzle the olive oil over the dough and squeeze and fold until fully absorbed. The dough is about 70% hydration and 2% salt.
6. Bulk rise covered at about 75°F (24°C) for 4 to 5 hours. During the first 2 hours, do 4 sets of stretch-and-folds 30 minutes apart; at the second set, spread the walnuts and cheddar over the dough and fold them in.
7. The dough is ready when it has grown about 50%, looks domed and jiggly, and shows bubbles at the sides.
8. Dust the counter with half of the reserved all-purpose flour, turn out the dough, pre-shape into a loose round, rest 20 minutes, then shape a tight boule, tucking any exposed cheese underneath.
9. Dust a basket or towel-lined bowl with the remaining half of the reserved all-purpose flour, set the boule in seam side up, cover, and cold proof in the fridge 10 to 14 hours overnight, until it springs back slowly when poked.
10. Preheat the oven to 425°F (220°C) with a rack in the middle; line a baking sheet with parchment.
11. Turn the boule onto the parchment, score the top, and bake 35 to 40 minutes until golden and 205°F (96°C) in the center.
12. Brush the hot loaf all over with the melted butter, then cover loosely with a clean towel to soften the crust.
13. Cool on a rack at least 2 hours before slicing into 12 slices.