Spelt Oat Porridge Walnut Sourdough Batard

Steps

1. Day 1, 8 a.m.: Feed your sourdough starter with its usual equal-weight feeding and leave it at about 75°F (24°C); it should peak, doubled and bubbly, in 4 to 6 hours.
2. Preparation, while the starter rises: toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes until fragrant, cool, then chop coarsely. Set aside 10 g of the oats for the topping.
3. Make the oat soaker: in a small saucepan simmer the remaining 35 g of oats with 80 g of the water for 2 to 3 minutes into a thick porridge, then cool to room temperature.
4. About 1 hour before the starter peaks, mix the whole spelt flour and the bread flour with 280 g of the water in a large bowl until no dry patches remain; cover and rest 45 minutes.
5. Add the peaked sourdough starter and the honey to the dough and pinch and fold until combined. This dough is about 76% hydration and 2% salt, counting all the water against the whole spelt flour and bread flour together.
6. Sprinkle the salt over the dough with the last 20 g of the water and squeeze it through until absorbed, then work in the cooled oat porridge and the toasted walnuts until evenly spread.
7. 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 with wet hands, 30 minutes apart, then leave the dough alone; it's ready when about 50% larger, domed and jiggly with bubbles on the sides.
8. Turn the dough onto a clean, lightly dampened counter, pre-shape into a loose round and rest 20 minutes. Shape into a long, snug batard so the slices come out an even sandwich size.
9. Mist the top of the batard with water and roll it in the reserved 10 g of oats, then place it oat side down in a parchment-lined oval basket or loaf pan.
10. Cover and cold-proof in the fridge for 12 to 16 hours overnight; spelt proofs quickly, so it's ready when it springs back slowly when poked.
11. Day 2: Put a Dutch oven with its lid in the oven and preheat to 475°F (245°C) for 45 minutes.
12. Turn the cold batard out oat side up onto parchment, score one long slash down its length, and lower it into the hot Dutch oven. Bake covered 20 minutes.
13. Uncover, lower to 450°F (230°C) and bake 25 to 30 minutes more, until the crust is deep mahogany brown and the center reads 205 to 210°F (96 to 99°C).
14. Cool the loaf on a rack for at least 2 hours before slicing into 12 slices.