Cheddar Walnut Oat Sourdough Boule

Steps

1. Day 1, about 8 am: feed your sourdough starter 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 peak is when you will use the sourdough-starter.
2. While the starter rises, pour 50 g of the water (one-seventh of the total) over the oats in a small bowl and let them soak at least 1 hour.
3. Toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes, stirring, until fragrant. Cool, then chop them roughly. Cut the cheddar into cubes and refrigerate it until needed.
4. When the starter peaks, stir the remaining 300 g of water and the sourdough-starter together in a large bowl. Add the bread flour and mix until no dry flour remains. Cover and rest 45 minutes.
5. Sprinkle the salt and the soaked oats over the dough. Squeeze and fold until fully combined. The dough is about 80% hydration and 2% salt, counting the flour and water in the starter; the oats absorb some of that water, so the dough handles like a moderate dough.
6. Bulk rise, covered, 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. At the second set, spread the dough out and scatter half the walnuts and half the cheddar over it, then fold it up. At the third set, add the remaining half of the walnuts and the remaining half of the cheddar the same way. Then leave the dough undisturbed until it has grown by about 50%, looks domed and jiggles with bubbles at the edges.
7. Turn the dough onto an unfloured counter, pre-shape it into a loose round and rest it 20 minutes. Then shape a tight boule, tucking any exposed cheese underneath.
8. Dust a towel-lined bowl or banneton with the cornmeal and set the boule in it seam side up. Cover and cold proof in the fridge for 12 to 16 hours, overnight.
9. Day 2: put a covered Dutch oven in the oven and preheat to 475°F (245°C) for 45 minutes. The cold dough is ready when it springs back slowly when poked.
10. Turn the boule onto parchment, score it with one confident slash and lower it into the hot Dutch oven. Cover and bake for 20 minutes.
11. Uncover, lower the heat to 450°F (230°C) and bake 22 to 25 minutes more, until the crust is deep brown and crisp and the center reads 205 to 210°F (96 to 99°C).
12. Cool the loaf on a rack for at least 2 hours before slicing into 12 slices, so the crumb sets and the cheese firms up.