Toasted Oat Porridge Cheddar Sourdough Batard

Steps

1. Day 1, about 8 a.m.: feed the sourdough starter with equal weights of flour and water from your own supply (separate from the recipe flour) and leave it at about 75°F (24°C) for 4 to 6 hours, until doubled, domed and bubbly, and a spoonful floats in water.
2. While the starter rises, make the porridge: toast 60 g of the oats (three-quarters) in a dry saucepan over medium heat for 3 to 4 minutes until golden and nutty. Add 120 g of the water, the ground flaxseed and the chia seeds, and stir over low heat for 2 to 3 minutes until thick. Spread on a plate and cool completely. Set aside the remaining 20 g of oats for the crust.
3. About 1 p.m., when the starter peaks: in a large bowl, stir 310 g of the water with the honey and the sourdough starter until milky. Add the flour and mix until no dry bits remain. Cover and rest 45 minutes.
4. Sprinkle the salt over the dough, add the remaining 20 g of water, and squeeze and fold until fully absorbed, about 3 minutes. The dough is about 76% hydration and 2% salt (counting the starter's flour and water, not the porridge).
5. Bulk rise at about 75°F (24°C) for 4 to 5 hours. After 30 minutes, spread the cooled oat, flaxseed and chia seed porridge, the cheddar and the black pepper over the dough and fold them in with a first set of stretch-and-folds. Do 3 more sets of stretch-and-folds 30 minutes apart, then let it rest untouched until about 50% larger, airy, and jiggly at the edges.
6. Turn the dough onto an unfloured counter, pre-shape into a loose round and rest 20 minutes. Shape into a tight batard, building surface tension; tuck any exposed cheddar underneath.
7. Mist the top lightly with water and roll it in the reserved 20 g of oats to coat. Place seam-side up in a floured, towel-lined oval basket.
8. Cover and cold-proof in the fridge for 12 to 16 hours overnight, until it springs back slowly when poked.
9. Day 2: put a Dutch oven with its lid in the oven and preheat to 475°F (245°C) for 45 minutes.
10. Turn the dough onto parchment, score one long slash down the length, and lower it into the hot Dutch oven. Bake covered 20 minutes.
11. Uncover, lower to 450°F (230°C) and bake 22 to 27 minutes more, until deep brown, crisp, and 205°F (96°C) in the center.
12. Cool the loaf on a rack for at least 2 hours before slicing into 12 slices, so the crumb sets and the crust stays crisp.