Seeded Oat Rye Sourdough Batard

Steps

1. About 6 hours before mixing (the evening before or first thing in the morning), feed your starter with equal weights of extra rye flour and water from your own supply, not from this recipe's amounts, and leave it at about 75°F (24°C) for 4 to 6 hours, until doubled, domed and bubbly.
2. Make the oat soaker while the starter rises: toast the pumpkin seeds and sesame seeds in a dry pan for 3 to 4 minutes until fragrant, then cool. In a bowl, pour the boiling portion of the water (100 g) over the soaker portion of the oats (60 g) and the flaxseed, stir, cover and let cool to room temperature.
3. When the starter peaks, stir the sourdough starter into the lukewarm portion of the water (380 g) in a large bowl until milky.
4. Add the rye flour and the bread flour and mix with a wet hand or stiff spatula until no dry patches remain. Cover and rest 30 minutes.
5. Add the salt, the dough portion of the olive oil (15 g), the rosemary, the oat soaker, and the toasted pumpkin seeds and sesame seeds. Squeeze and fold the dough until everything is evenly mixed; it will be sticky, as rye doughs are. This is about 86% hydration and 2% salt, counting the starter but not the soaker.
6. Bulk rise, covered, at about 75°F (24°C) for 3 to 4 hours. During the first 2 hours do 4 sets of stretch-and-folds 30 minutes apart, using wet hands. Bulk is done when the dough has risen about 50%, looks puffy and shows bubbles on the surface and sides.
7. Dust a work surface lightly with a little extra rye flour. Turn out the dough and shape it into a batard by folding the sides to the middle and rolling it up into a tight log.
8. Brush the top with half of the brushing olive oil (5 g) and press the topping portion of the oats (10 g) over it. Place it seam side up in an oval basket or loaf pan lined with a cloth dusted with rye flour.
9. Cover and cold proof in the fridge overnight, 12 to 14 hours. The dough is ready when it has risen slightly and springs back slowly when poked.
10. Put a Dutch oven with its lid in the oven and preheat to 475°F (246°C) for 45 minutes.
11. Turn the dough onto parchment, oat side up, and score one long, shallow slash down its length. Lower it into the hot Dutch oven, cover and bake 20 minutes.
12. Remove the lid, lower the heat to 450°F (232°C), and bake 25 to 30 minutes more, until the crust is deep brown and crisp and the center reads 205 to 210°F (96 to 99°C).
13. Brush the hot crust with the remaining brushing olive oil (5 g) for a focaccia-style sheen. Cool the loaf on a rack at least 3 hours before slicing, as the rye crumb sets as it cools, then cut into 12 slices.