Prepare Your Starter: Feed your starter 4–6 hours before mixing. It should double or triple in volume, be bubbly and elastic, and smell lightly tangy and sweet.
Mix the Dough: In a large bowl, combine active starter, milk, sugar, and lemon zest. Stir until well blended. Add flour and salt. Mix until a shaggy dough forms. Let rest for 20–30 minutes. Knead in the softened butter until fully incorporated.
Knead Until Elastic: Knead by hand for 8–10 minutes or in a mixer for 5–6 minutes until the dough is smooth and elastic.
Bulk Fermentation: Place the dough in a lightly greased bowl and cover. Let ferment at room temperature for 4–6 hours, or until nearly doubled.
Prepare the Fillings: In a small bowl, mix cream cheese, sugar, lemon juice, and vanilla until smooth. Toss blueberries with flour.
Shape and Fill: Turn the dough onto a lightly floured surface. Roll into a rectangle, spread the cream cheese mixture, scatter blueberries, and roll tightly into a log.
Final Proof: Place the shaped loaf seam-side down into a parchment-lined loaf pan or banneton. Cover and let proof for 2–4 hours.
Bake: Preheat oven to 375°F / 190°C. Bake for 45–50 minutes, tenting with foil after 30 minutes if necessary.
Cool Completely: Remove from pan and cool on a rack for at least 90 minutes.
