Feed the sourdough starter 4-6 hours before baking until bubbly and active.

In a large bowl, combine sourdough starter, bread flour, warm water, sugar, salt, lemon juice, and lemon zest. Stir until a shaggy dough forms. Cover and rest for 30 minutes.

With wet hands, perform a series of stretch-and-folds: lift one side of the dough, fold it over itself, rotate, and repeat. Do this four times per session, three times total, resting 30 minutes between each.

After the final fold, cover the dough and let it ferment at room temperature for 3-6 hours, until doubled and bubbly.

Refrigerate overnight (cold fermentation) for 8-12 hours to develop flavor and structure.

The next day, line a 9×13-inch baking pan with parchment paper and drizzle olive oil evenly over the bottom.

Gently transfer the dough to the prepared pan. Using oiled fingers, press and stretch the dough to fill the pan. Cover and let it rise for 4-5 hours until puffy.

Preheat oven to 400°F (200°C).

Sprinkle blueberries evenly over the dough, drizzle melted butter, and dimple the surface with your fingertips.

Bake for 30-40 minutes, until the top is light golden brown and the internal temperature reaches 200°F (93°C).

Cool in the pan for 10-15 minutes, then transfer to a wire rack.

In a small bowl, whisk powdered sugar with lemon juice until smooth and pourable.

Drizzle the lemon glaze over cooled focaccia. Slice and serve warm or at room temperature.