Feta Walnut Oregano Focaccia-Style Sourdough Boule

Steps

1. Day 1, 8:00 am: Feed your sourdough starter and leave it at about 75°F (24°C); it should peak, doubled and bubbly with a domed top, 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, cool, and chop. Crumble the feta, pat it dry, and mince the garlic.
3. Set aside about 2 tablespoons of the bread flour in a small dish for dusting later; the rest goes into the dough.
4. About 12:00 pm, once the starter peaks: in a large bowl, stir the remaining bread flour and the whole wheat flour with the water until no dry streaks remain. Cover and rest 45 minutes (autolyse).
5. Add the sourdough starter to the dough and squeeze and fold it in by hand for 2 to 3 minutes. Sprinkle on the salt and two-thirds of the olive oil, then squeeze and fold for 3 more minutes until the dough feels smooth and absorbs the oil. This is about 72% hydration and 1.8% salt, kept a little low because the feta is salty.
6. Cover and rest 30 minutes at about 75°F (24°C). Stretch the dough out on a wet counter into a rough rectangle, scatter over the walnuts, feta, oregano and garlic, then roll it up and return it to the bowl.
7. Bulk rise at about 75°F (24°C) for 4 to 5 hours total. During the first 2 hours, do 3 sets of stretch-and-folds 30 minutes apart, then leave the dough alone until it has grown about 50%, looks domed and jiggly, and shows bubbles on the sides.
8. Dust the counter with half of the reserved bread flour, turn the dough onto it, pre-shape it into a loose round, and rest 20 minutes uncovered.
9. Dust a proofing basket or towel-lined bowl with the remaining reserved bread flour. Shape the dough into a tight boule, building surface tension, and place it seam side up in the basket.
10. Cover and cold proof in the fridge for 12 to 14 hours overnight.
11. Day 2: Put a Dutch oven with its lid in the oven and preheat to 475°F (245°C) for 45 minutes.
12. Turn the cold dough onto parchment, brush the top with the remaining one-third of the olive oil, and score a deep slash or cross with a sharp blade.
13. Lower the loaf on its parchment into the hot Dutch oven, cover, and bake 20 minutes. Uncover, lower the heat to 450°F (230°C), and bake 22 to 25 minutes more until deep golden and crisp, and the center reads 205 to 210°F (96 to 99°C).
14. Cool the boule on a rack for at least 2 hours before slicing into 12 slices, so the crumb sets and the crust stays crisp.