Caramelized Onion Walnut Rye Focaccia Boule

Steps

1. Starter, about 6 hours ahead: give your sourdough starter its usual 1:1 feeding from your own starter supply (separate from this recipe's ingredients) and leave it at about 75°F (24°C) for 4 to 6 hours, until doubled, domed and bubbly.
2. While the sourdough starter rises, toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes until fragrant, then cool and chop.
3. In the same skillet, warm one-quarter of the olive oil over medium-low heat, add the onion and cook, stirring often, for 20 to 25 minutes until deep golden and soft. Stir in the honey and half of the thyme, then cool completely.
4. In a large bowl, whisk the peaked sourdough starter into the water. Add the rye flour and the bread flour and mix until no dry rye flour or bread flour remains. Cover and rest 30 minutes.
5. Sprinkle the salt over the dough, add one-third of the olive oil, and squeeze and fold with wet hands until absorbed. Counting the sourdough starter, the dough is about 75% hydration and 2% salt.
6. Bulk rise at about 75°F (24°C) for about 4 hours. During the first 2 hours do 3 sets of gentle stretch-and-folds 30 minutes apart, adding the walnuts and half of the cooled onions during the first set. Rye dough is sticky, so use wet hands. Bulk is done when the dough has risen by about half and looks puffy with bubbles on the sides.
7. Turn the dough onto a lightly wet counter and shape it into a round boule with wet hands. Set it on a sheet of parchment in a bowl, cover, and refrigerate overnight, 10 to 14 hours, for a cold proof.
8. In the morning, put a covered Dutch oven in the oven and preheat to 450°F (230°C) for 45 minutes.
9. Take out the dough; it should be slightly puffed and spring back slowly when poked. Drizzle with the remaining olive oil except about 1 teaspoon, press deep dimples all over with oiled fingertips, then scatter the remaining onions and the remaining thyme on top.
10. Lift the dough by the parchment into the hot Dutch oven, cover and bake 35 minutes. Uncover, lower to 425°F (220°C) and bake 10 to 15 minutes more, until golden and the center reads 205–210°F (96–99°C).
11. For a soft crust, brush the hot loaf with the reserved teaspoon of olive oil and loosely tent it with a clean towel as it cools.
12. Cool on a rack for at least 2 hours before slicing into 12 slices, so the rye crumb can set.