Rosemary Parmesan Sourdough Sandwich Batard

Steps

1. Day 1, about 8 a.m.: give your sourdough starter its usual 1:1:1 feeding by weight and leave it at about 75°F (24°C). It should peak, doubled, domed and bubbly, in 4 to 6 hours; use the sourdough starter for this loaf at that peak.
2. About 12:30 p.m.: in a large bowl, mix the bread flour and the whole wheat flour with the first 340 g of the water until no dry patches remain. Cover and rest 45 minutes (autolyse).
3. Add the peaked sourdough starter to the dough and squeeze it through with a wet hand for 2 minutes. Sprinkle on the salt with the remaining 20 g of the water and squeeze again until absorbed. Then add 15 g of the olive oil and fold until the dough takes it in. This dough is about 72% hydration and 2% salt by bakers' percentages.
4. Bulk rise, covered, at about 75°F (24°C) for 4 to 5 hours. After 30 minutes, do the first set of stretch-and-folds: stretch one side up and fold it over, turning the bowl four times. During this first set, scatter the Parmesan, rosemary and black pepper over the dough and fold them in.
5. Do three more sets of stretch-and-folds 30 minutes apart, then leave the dough alone for the rest of the bulk rise. It is ready when it has grown about 50%, looks domed and airy, and jiggles when the bowl is shaken.
6. Turn the dough onto a clean counter, round it loosely with damp hands and rest it uncovered 20 minutes.
7. Shape a tight batard: fold the sides in, then roll it up firmly from the top into an oval log, pinching the seam. Place it seam-side up in an oval bowl or banneton lined with a clean, dry towel.
8. Cover and cold proof in the fridge for 12 to 14 hours, overnight.
9. Day 2, morning: place a Dutch oven with its lid in the oven and preheat to 475°F (245°C) for 45 minutes.
10. Turn the cold dough onto parchment, brush the top with the remaining 5 g of the olive oil, and slash one long, shallow cut down its length at a low angle.
11. Lower the dough on its parchment into the hot Dutch oven, cover and bake 20 minutes. Remove the lid, lower the oven to 450°F (230°C) and bake 20 to 25 minutes more, until deep golden and crisp and the center reads 205°F (96°C).
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.