In a large bowl, combine the sourdough starter, bread flour, salt, and sugar. Mix until the dough comes together in a shaggy mass.
Gently fold in the blueberries and lemon zest.
Let the dough rest and rise for 4-5 hours, or until it has doubled in size.
In a separate bowl, mix the softened cream cheese, granulated sugar, and vanilla extract.
Roll out the dough into a rectangle, about ¼ inch thick. Spread the cream cheese mixture over the dough, leaving a 1-inch border around the edges.
Fold the dough over the cream cheese mixture, and gently press the edges to seal.
Shape the dough into a round or oblong loaf.
Let the bread rest and rise for another 2-3 hours, or until it has doubled in size again.
Preheat your oven to 375°F (190°C). Brush the top of the bread with beaten egg for a golden glaze. Bake for 35-40 minutes, or until the bread is golden brown and sounds hollow when tapped.
