Soft Sharp Cheddar Mustard Spelt Boule

Steps

1. Feed your sourdough starter about 8 a.m. on day one with its usual 1:1 feeding, and leave it at about 75°F (24°C) for 4 to 6 hours until it has doubled, is domed and bubbly, and a spoonful floats in water.
2. While the starter rises, cut the cheddar into cubes and refrigerate it so it stays in pieces when folded in.
3. When the starter peaks, whisk the water, milk and mustard together in a large bowl, then stir in the sourdough starter until milky.
4. Add the whole spelt flour and the bread flour and mix by hand until no dry patches remain. Cover and rest 30 minutes.
5. Sprinkle the salt and black pepper over the dough and squeeze and fold them in for 2 to 3 minutes until evenly mixed. This dough is about 70% hydration counting the milk and starter, and about 2% salt.
6. Bulk rise covered at about 75°F (24°C) for 4 to 5 hours. After the first 30 minutes, stretch the dough out on a damp counter, scatter half of the cheddar over it and roll it up; 30 minutes later, on the second set of stretch-and-folds, fold in the remaining half of the cheddar the same way. Give two more gentle sets of stretch-and-folds 30 minutes apart, then leave the dough undisturbed until it has grown by about 50%, looks domed and jiggles. Spelt is delicate, so handle it gently.
7. Turn the dough onto a lightly dampened counter, pre-shape into a loose round, and rest 20 minutes. Shape into a tight boule, tucking any cheese that pokes out back underneath, and place seam side up in a basket or bowl lined with a clean towel.
8. Cover and cold proof in the fridge overnight, 12 to 14 hours.
9. The next morning, line a baking sheet with parchment and preheat the oven to 400°F (200°C) for 45 minutes; the lower heat and open baking keep the crust soft.
10. Turn the cold boule onto the parchment, score a shallow cross on top, and bake 40 to 45 minutes until deep golden and the center reads 190°F (88°C) on an instant-read thermometer.
11. Brush the hot loaf all over with the melted butter, then drape a clean kitchen towel over it to soften the crust.
12. Cool the loaf on a rack for at least 2 hours before slicing into 12 slices, so the crumb and cheese set.