Soft Rosemary Oat Country Sourdough Boule

Steps

1. Day 1, 8 a.m.: feed your starter with equal weights of bread flour and water from your usual starter supply, separate from this recipe's amounts. At about 75°F (24°C) it peaks, doubled and bubbly, in 4 to 6 hours, around noon to 2 p.m. Use the sourdough starter at its peak.
2. Day 1, 11 a.m.: toast 70 g of the oats in a dry skillet over medium heat for 4 to 5 minutes until golden and fragrant. Pour the 100 g of hot water over the toasted oats in a bowl, stir and let this oat soaker cool for 1 hour.
3. Day 1, noon: in a large bowl, mix the bread flour and the whole wheat flour with the 230 g of lukewarm water and the milk until no dry bits remain. Cover and rest 45 minutes (autolyse).
4. Add the peaked sourdough starter, the salt and 15 g of the olive oil; squeeze and fold until fully combined, about 3 minutes. Counting the soaker water and the milk, this is about 81% hydration and 2% salt.
5. Rest the dough 30 minutes, then add the cooled oat soaker and the rosemary, folding and pinching until evenly spread.
6. Bulk rise at about 75°F (24°C) for 4 to 5 hours: do 4 sets of stretch-and-folds with wet hands 30 minutes apart in the first 2 hours, then leave it covered until risen by about 50%, domed and jiggly with bubbles at the edges.
7. Turn the dough onto a clean, lightly dampened counter, pre-shape into a loose round and rest 20 minutes. Shape into a tight boule.
8. Brush the top with half of the remaining 5 g of olive oil and press the remaining 20 g of oats onto it. Set it oat side down in a bowl lined with a lightly oiled sheet of parchment.
9. Cover and cold-proof in the fridge overnight, 12 to 16 hours, until puffy and it springs back slowly when poked.
10. Day 2, morning: place a lidded Dutch oven in the oven and preheat to 450°F (230°C) for 45 minutes.
11. Turn the boule out oat side up onto parchment, score the top, lower it into the hot Dutch oven and cover. Bake 20 minutes covered.
12. Uncover, reduce to 400°F (205°C) and bake 15 to 20 minutes more, until lightly golden and 205°F (96°C) in the center. The lower uncovered heat keeps the crust soft.
13. Brush the hot crust with the last half of the 5 g of olive oil, drape loosely with a clean towel to soften the crust, and cool on a rack at least 2 hours before cutting into 12 slices.