Caramelized Onion Cheddar Oat Rye Boule

Steps

1. Night before (about 8 to 12 hours ahead): give your sourdough starter its usual feeding, separate from this recipe, and leave it at about 75°F (24°C); it should peak, doubled and bubbly, in 6 to 8 hours. Weigh out the sourdough starter for the dough at its peak.
2. Make the oat soaker: toast the 70 g portion of oats in a dry skillet over medium heat for 4 to 5 minutes until golden and nutty. Tip into a bowl, pour over the 100 g boiling portion of water, stir, cover and cool to room temperature, about 1 hour.
3. Caramelize the onion: warm the olive oil in the skillet over medium-low heat, add the onion and cook, stirring often, for 25 to 30 minutes until deep golden and jammy. Stir in the thyme and cool completely.
4. Once the starter peaks, mix the dough: in a large bowl stir the 380 g lukewarm portion of water with the sourdough starter, then add the rye flour and bread flour and mix until no dry patches remain. Cover and rest 45 minutes.
5. Sprinkle the salt over the dough, squeeze it in with wet hands, then fold in the cooled oat soaker until evenly mixed. The dough will be sticky and paste-like, which is normal for a rye-heavy loaf; this is about 76% hydration and 2.2% salt (dough water and salt measured against the combined rye and bread flours, soaker water extra).
6. Bulk rise at about 75°F (24°C) for 3 1/2 to 4 1/2 hours. During the first 2 hours do 3 sets of gentle stretch-and-folds with wet hands, 30 minutes apart; spread the caramelized onion and cheddar over the dough before the second set and fold them in. Bulk is done when the dough has risen about 50%, looks domed and shows bubbles at the edges.
7. Shape: turn the dough onto a lightly wet counter, fold the edges to the center and roll into a tight round boule. Mist the top with water and roll it in the 20 g portion of oats to coat.
8. Proof: set the boule oat side down in a bowl lined with parchment, cover and refrigerate 12 to 14 hours for an overnight cold proof. It is ready when it has puffed slightly and springs back slowly when poked.
9. Put a Dutch oven with its lid in the oven and preheat to 475°F (245°C) for 45 minutes.
10. Turn the cold boule onto a sheet of parchment, oat side up, and score a cross about 1/2 inch deep. Lower it on the parchment into the hot Dutch oven, cover and bake 25 minutes.
11. Remove the lid, lower the heat to 435°F (225°C) and bake 20 to 25 minutes more until deep brown and crisp and the center reads 205 to 210°F (96 to 99°C).
12. Cool the loaf on a rack at least 3 hours before slicing so the rye crumb sets, then cut into 12 slices.