Soft Oat and Walnut Sourdough Boule

Steps

1. Day 1, morning: feed your sourdough starter with equal weights of flour and water and leave it at about 75°F (24°C) for 4 to 6 hours, until doubled, domed and bubbly. This fed starter is the only leavening.
2. While the starter rises, make the oat soaker: pour 120 g of the water, boiling, over 80 g of the oats, stir, cover and cool to room temperature, about 1 hour.
3. Toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes until fragrant, then cool and chop them.
4. When the starter has peaked, stir it into a large bowl with the remaining 200 g of water (lukewarm), the milk and the honey until milky.
5. Add the flour and mix until no dry flour remains. Cover and rest 45 minutes.
6. Add the salt, the cooled oat soaker and 20 g of the olive oil (two-thirds), squeezing and folding until the dough is smooth and the oats are evenly spread. This dough is about 64% hydration before the soaker and about 2% salt, counting the starter's flour and water.
7. Bulk rise 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, adding the walnuts with the first set.
8. The bulk rise is done when the dough has grown by about half, feels airy and shows bubbles at the edges. Turn it onto a lightly oiled counter and pre-shape into a loose round; rest 20 minutes.
9. Shape a snug boule, building surface tension. Roll the top in the remaining 10 g of oats and set it seam-side up in a towel-lined bowl.
10. Cover and cold-proof in the fridge for 12 to 14 hours overnight.
11. Day 2: heat the oven to 400°F (200°C) with a rack in the middle. Turn the boule onto a parchment-lined baking sheet and brush it with half of the remaining olive oil (5 g). Score it shallowly.
12. Bake 35 to 40 minutes, until golden and the center reads 190°F (88°C), since the dough is enriched with milk and oil.
13. Brush the hot loaf with the last of the olive oil (5 g) and drape it loosely with a clean towel for 15 minutes to soften the crust.
14. Move the loaf to a rack and cool at least 1½ hours before slicing into 12 slices.