Parmesan Garlic Oat Porridge Sourdough Batard

Steps

1. Day 1, morning: feed your sourdough starter with equal weights of flour and water and keep it at about 75°F (24°C); it should peak, doubled and bubbly, in 4 to 6 hours.
2. While the starter rises, toast 85 g of the oats in a dry small pot over medium heat for 3 minutes until fragrant, add the water and simmer 3 to 4 minutes into a thick porridge, then spread on a plate to cool completely.
3. In a small skillet, warm 10 g of the olive oil over low heat, add the garlic and cook 1 to 2 minutes until soft and pale gold, then stir in the rosemary and cool.
4. When the starter peaks, whisk the buttermilk and the sourdough starter together in a large bowl.
5. Add the flour and mix until no dry bits remain, then cover and rest 30 minutes.
6. Sprinkle the salt over the dough, add the cooled oat porridge, the garlic-rosemary oil, 30 g of the Parmesan and 20 g of the olive oil, and squeeze and fold until evenly worked in, about 5 minutes. The dough is about 60% hydration (counting buttermilk and starter water) and 1.6% salt before the porridge, which makes it soft and tacky; the Parmesan adds more salt.
7. Bulk rise covered at about 75°F (24°C) for 4 to 5 hours, doing 4 sets of stretch-and-folds 30 minutes apart in the first 2 hours. It's ready when grown about 50%, domed, jiggly and bubbly on the sides.
8. Turn the dough onto a lightly floured counter, pre-shape into a loose round, rest 20 minutes, then shape a tight batard and set it seam side up in a floured towel-lined bowl or banneton.
9. Cover and cold proof in the fridge overnight, 12 to 14 hours.
10. Day 2: line a sheet pan with parchment and preheat the oven to 400°F (200°C) for 30 minutes.
11. Turn the batard onto the parchment, brush with the remaining 10 g of olive oil, scatter over the remaining 15 g of oats and the remaining 10 g of Parmesan, pressing lightly, and score one long slash.
12. Bake uncovered for 35 to 40 minutes, until deep golden and at least 190°F (88°C) in the center; baking open with the oil and buttermilk keeps the crust soft.
13. Cool the loaf on a rack for at least 1 hour before cutting into 12 slices.