In a large bowl, mix bread flour, sourdough starter, warm water, sugar, salt, and lemon zest.
Stir until a shaggy dough forms. Cover and let rest for 30 minutes (autolyse).
Knead the dough for 8–10 minutes until smooth and elastic.
Place in a greased bowl, cover, and let rise at room temperature for 2–3 hours or until doubled in size.
In a medium bowl, beat cream cheese, powdered sugar, and vanilla until smooth. Set aside.
Once risen, punch down the dough and roll it out into a rectangle.
Spread the cream cheese mixture evenly over the surface.
Sprinkle with blueberries.
Carefully roll the dough into a log, sealing the edges.
Place into a greased loaf pan, seam side down. Cover and let rise again for 1–2 hours.
Preheat oven to 375°F (190°C).
Brush the loaf with egg wash for a golden crust.
Bake for 40–45 minutes until golden brown and cooked through.
Let the loaf cool in the pan for 10 minutes before transferring to a wire rack.
Slice and enjoy warm or at room temperature.
