Rosemary Garlic Walnut Seeded Sourdough Boule

Steps

1. About 12 hours before mixing, feed your starter with its usual equal-weight refreshment and leave it at about 75°F (24°C); it should peak, doubled and bubbly, in 4 to 12 hours. If your starter peaks quickly, feed it 4 to 6 hours before mixing instead.
2. Toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes until fragrant, then cool and chop. Toast the sesame seeds in the same skillet for 2 minutes and cool.
3. Warm half the olive oil in the skillet over low heat, add the garlic and rosemary, and cook gently for 1 to 2 minutes until the garlic is soft but not browned; cool completely.
4. Set aside 10 g of the all-purpose flour for dusting. In a large bowl, stir 270 g of the water, the milk and the starter until milky. Add the remaining 440 g of all-purpose flour and the whole wheat flour and mix until no dry bits remain. Cover and rest 45 minutes.
5. Sprinkle the salt over the dough with the remaining 20 g of water and squeeze it in until absorbed. The dough is about 71% hydration (counting the water in the starter and milk) and 2% salt.
6. Add the cooled garlic-rosemary oil, the walnuts, the sesame seeds and the flaxseed, and squeeze and fold until evenly spread.
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. The dough is ready when it has grown about 50%, feels airy and domed, and shows bubbles on the sides.
8. Dust the counter with half of the reserved all-purpose flour, pre-shape the dough into a loose round, rest 20 minutes, then shape a tight boule. Dust a bowl or banneton with the rest of the reserved all-purpose flour and set the boule in it seam side up.
9. Cover and cold-proof in the fridge for 12 to 16 hours, until it springs back slowly when poked.
10. Preheat the oven to 425°F (220°C) with a rack in the middle for 30 minutes. Turn the boule onto a parchment-lined baking sheet, score it, and brush it with the remaining half of the olive oil.
11. Bake 20 minutes, then lower the heat to 375°F (190°C) and bake 20 to 25 minutes more, until golden and the center reads 190°F (88°C), since the dough is enriched with milk and oil.
12. Wrap the hot loaf loosely in a clean towel for 30 minutes to soften the crust, then unwrap and cool fully on a rack, about 2 hours, before slicing into 12 slices.