Mix 6 g active starter with 60 g flour and 60 g water. Cover and keep at 78–80°F for 10–12 hours until bubbly and doubled.

In a large bowl combine 120 g ripe levain and 340 g water. Stir to dissolve. Add 500 g bread flour and 10 g salt. Mix until a wet, sticky dough forms. Cover and rest for 30 minutes.

Perform stretch-and-folds every 30 minutes for 1½ hours, approximately three sets.

Spread or press cold butter into the dough after the final fold. Fold to encase the butter and continue gentle folds until mostly incorporated. Sprinkle in about 150 g of shredded Gruyère during folding.

Cover the dough and let it rise for 6–7 hours at room temperature until about 70% risen with visible bubbles.

Turn dough onto a floured surface. Gently stretch into a roughly rectangular sheet. Evenly scatter the diced ham and remaining cheese, leaving a margin. Fold over and roll into a loaf shape. Seal seams.

Place shaped loaf in a banneton or lined pan, cover, and refrigerate overnight or 12–18 hours.

Preheat a Dutch oven to 450°F. Score the cold dough, sprinkle with reserved Gruyère, and transfer to the preheated Dutch oven. Cover and bake for 25 minutes, then uncover and reduce to 425°F. Bake another 20 minutes until deep golden and internal temperature reaches 200–205°F.

Remove loaf and cool on a rack for at least 1 hour before slicing.