Make the Dough:

In a large bowl, mix sourdough starter and water until combined.

Add bread flour and mix until just combined. Let rest for 30 minutes (autolyse).

Add salt and lemon zest. Gently knead in the bowl until incorporated.

Bulk Fermentation:

Let the dough rest at room temperature for 4–6 hours, covered.

During the first 2 hours, perform stretch and folds every 30 minutes.

On the last fold, gently add in the blueberries without overmixing.

Shape & Cold Proof:

Shape the dough into a boule or oval on a floured surface.

Transfer to a floured banneton or towel-lined bowl.

Cover and refrigerate overnight (8–12 hours).

Bake:

Preheat oven to 450°F (232°C) with a Dutch oven inside.

Turn dough out onto parchment paper, score the top, and place in the Dutch oven.

Bake covered for 20 minutes, then uncovered for 20–25 minutes until golden brown.

Cool completely before slicing.

Whipped Blueberry Butter:

In a small bowl, combine softened butter, mashed blueberries, honey, and salt.

Mix or whip until fluffy and combined. Chill until ready to serve.