Maple Oat Porridge Rosemary Sourdough Boule

Steps

1. Day 1, about 9 a.m.: feed your sourdough starter with equal weights of flour and water from your own supply (not the recipe amounts) and keep it at about 75°F (24°C); it should double and peak, domed and bubbly, in 4 to 6 hours.
2. Meanwhile make the porridge: in a small pot, simmer 110 g of the oats (the larger portion) with the whole milk, stirring, for 5 minutes until thick and creamy. Off the heat, stir in the maple syrup and the rosemary. Spread on a plate and cool completely to room temperature.
3. About 1 p.m., when the starter peaks: in a large bowl, stir the 235 g portion of the water with the sourdough starter until milky, add the flour, and mix until no dry flour remains. Cover and rest 30 minutes.
4. Sprinkle the salt over the dough, add the remaining 25 g of the water, and squeeze and fold until absorbed. The dough is about 69% hydration and 2% salt, not counting the porridge.
5. Add the cooled maple oat porridge and 20 g of the olive oil (two-thirds) and pinch and fold for 3 to 4 minutes until evenly mixed and the oil is absorbed; the dough will be soft and sticky.
6. Bulk rise, covered, at about 75°F (24°C) for 4 to 5 hours. In the first 2 hours, do 4 sets of stretch-and-folds 30 minutes apart. Bulk is done when the dough has grown about 50%, is domed and jiggly, and shows bubbles at the sides.
7. Turn the dough onto a lightly floured 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 10 g of olive oil, then roll the top in the remaining 30 g of oats to coat. Place oat-side down in a floured, towel-lined bowl.
9. Cover and cold proof in the fridge for 12 to 16 hours overnight.
10. Day 2: put a lidded Dutch oven in the oven and preheat to 475°F (245°C) for 45 minutes.
11. Turn the dough out onto parchment, oat side up. Brush with the last half of the olive oil, press 6 or 7 deep dimples into the top with oiled fingertips, and score one shallow slash around the side.
12. Lower the dough on its parchment into the hot Dutch oven, cover, and bake 20 minutes. Uncover, reduce to 425°F (220°C), and bake 25 to 30 minutes more until the crust is deeply browned and the center reads at least 190°F (88°C), since the milk enriches the dough.
13. Cool the loaf on a rack for at least 2 hours before slicing into 12 slices.