# What You'll Need:
→ Dough
01 - 2 3/4 cups bread flour
02 - 1/3 cup active sourdough starter
03 - 1/4 cup granulated sugar
04 - 1 large egg
05 - 1/2 cup whole milk, lukewarm
06 - 1/4 cup unsalted butter, softened
07 - 1 teaspoon fine sea salt
08 - Zest of 1 large lemon
→ Blueberry Swirl
09 - 1 1/2 cups fresh or frozen blueberries
10 - 1/4 cup granulated sugar
11 - 1 tablespoon lemon juice
12 - 1 tablespoon cornstarch
→ Crumble Topping
13 - 1/3 cup all-purpose flour
14 - 2 tablespoons unsalted butter, cold and cubed
15 - 2 tablespoons light brown sugar
16 - Pinch of salt
# How-To Steps:
01 - In a stand mixer bowl, combine bread flour, granulated sugar, fine sea salt, and lemon zest.
02 - Add the sourdough starter, egg, and lukewarm milk to the dry ingredients. Mix to form a shaggy dough.
03 - With the mixer on low speed, add softened butter in small pieces, mixing until fully incorporated and dough becomes smooth and elastic, approximately 8 to 10 minutes.
04 - Place dough in a lightly greased bowl, cover with a damp cloth, and let rise in a warm place until doubled in volume, about 2 hours.
05 - In a small saucepan, combine blueberries, granulated sugar, and lemon juice. Cook over medium heat until berries burst, approximately 5 minutes. Stir in cornstarch and simmer until thickened, 2 to 3 minutes. Cool completely.
06 - In a mixing bowl, combine all-purpose flour, light brown sugar, and salt. Cut in cold cubed butter using a pastry cutter or fork until the mixture resembles coarse breadcrumbs. Refrigerate until needed.
07 - Once dough has risen, gently punch down and roll out on a floured surface to a 10 by 14-inch rectangle.
08 - Spread the cooled blueberry mixture evenly over the dough, leaving a 1-inch border on all edges.
09 - Roll up the dough tightly from the short side to form a log. Place seam-side down in a greased 9 by 5-inch loaf pan.
10 - Cover the loaf pan and let rise in a warm place until puffy, approximately 1 hour.
11 - Preheat oven to 350°F. Brush loaf with milk or beaten egg if desired, and sprinkle evenly with prepared crumble topping. Bake for 50 to 55 minutes, tenting with aluminum foil after 30 minutes if browning too quickly.
12 - Cool loaf in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.
