In a large mixing bowl, whisk together warm water, sourdough starter, and sugar until the mixture is smooth and the starter is dissolved.

Add in the flour, salt, melted butter, egg, lemon zest, and lemon juice. Stir everything together until a shaggy dough forms.

Knead it by hand for 8–10 minutes (or with a stand mixer on medium speed for 6–7 minutes) until the dough becomes smooth, slightly tacky but not sticky, and elastic.

Transfer the dough to a clean, lightly oiled bowl, cover with plastic wrap or a damp towel, and allow it to rest in a warm spot. The dough should rise for 4 to 5 hours, or until it has doubled in volume.

While the dough is proofing, prepare the filling. In a medium bowl, beat the softened cream cheese until smooth and lump-free.

Add the sugar, egg, and vanilla extract, and continue beating until the mixture is silky and creamy.

Once the dough has risen, gently punch it down to release air bubbles and divide it into two equal parts.

On a floured surface, roll each piece into a rectangle roughly ¼ inch thick.

Spread half of the cream cheese filling evenly over one of the rectangles, making sure to leave about a 1-inch border around the edges.

Scatter a generous handful of blueberries over the cream cheese layer, pressing them in lightly.

Cover with the second rectangle of dough, press the edges together firmly to seal, and gently shape the filled dough into a round or oval loaf.

Transfer the filled dough to a parchment-lined baking sheet. Cover loosely and let rest for 30–45 minutes while the oven preheats.

In a small bowl, whisk together the lemon juice and honey until the mixture is smooth and slightly thickened.

Brush this glaze gently over the entire surface of the loaf.

Preheat your oven to 375°F (190°C). Once the dough has rested, place it in the oven and bake for 35 to 40 minutes.

Let the loaf cool on a wire rack for at least 15 minutes before slicing.