Soft Rye Boule with Walnuts and Parmesan

Steps

1. The night before (about 8 pm), feed your sourdough starter from its own separate supply with equal weights of fresh rye flour and water, and leave it at about 75°F (24°C); it should peak, doubled and bubbly, in 4 to 8 hours, so mix the dough the next morning. The starter feeding is not taken from the recipe's rye flour.
2. While the starter rises, toast the walnuts in a dry skillet over medium heat for 4 to 5 minutes until fragrant, cool, and chop roughly.
3. In a large bowl, stir the water, milk, honey and sourdough starter until the starter dissolves.
4. Set aside about 10 g of the rye flour for dusting. Add the remaining rye flour and all the bread flour to the bowl and mix into a sticky, shaggy dough; cover and rest 30 minutes.
5. Sprinkle the salt and black pepper over the dough and squeeze them in with wet hands. This dough is about 76% hydration (water and milk) and 1.8% salt; rye dough feels paste-like, so work with wet hands rather than adding more rye flour or bread flour.
6. Add the Parmesan and walnuts and fold them in until evenly spread.
7. Bulk rise covered at about 75°F (24°C) for 3 to 4 hours, doing 3 sets of gentle stretch-and-folds 30 minutes apart in the first 90 minutes; it's ready when it has risen by about half and shows bubbles on top.
8. Dust the counter with half of the reserved rye flour, turn the dough out, and shape a round boule with wet hands.
9. Dust a banneton or towel-lined bowl with the remaining half of the reserved rye flour and place the boule in it seam side up.
10. Cover and cold proof in the fridge overnight, 10 to 14 hours.
11. Place a Dutch oven with its lid in the oven and preheat to 450°F (230°C) for 45 minutes.
12. Turn the cold boule onto parchment, score a shallow cross, lower it into the hot Dutch oven, cover and bake 25 minutes.
13. Uncover, lower the oven to 425°F (220°C) and bake 20 to 25 minutes more, until deep brown and 205°F (96°C) in the center.
14. Brush the hot crust with the melted butter right away to soften it, then cool the bread completely on a rack, at least 3 hours, before slicing into 12 slices; rye needs this time to set.