1. 1. Prepare your baking pan: Preheat your oven to 350 F (175 C). Lightly grease and flour a 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the sides to easily lift the loaf out later.

2. 2. Combine wet ingredients: In a large mixing bowl, whisk together the active sourdough starter, granulated sugar, melted butter, egg, and vanilla extract until well combined and smooth. The mixture should look creamy and slightly pale.

3. 3. Mix dry ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Make sure there are no lumps.

4. 4. Combine wet and dry: Gradually add the dry ingredient mixture to the wet ingredients, alternating with the milk. Begin and end with the dry ingredients. Mix until just combined. Be careful not to overmix; a few small lumps are okay and help keep the loaf tender.

5. 5. Add lemon and blueberries: Gently fold in the lemon zest. In a small bowl, toss the blueberries with the extra tablespoon of flour. This step helps prevent them from sinking to the bottom of the loaf during baking. Carefully fold the floured blueberries into the batter until evenly distributed.

6. 6. Bake the loaf: Pour the batter into the prepared loaf pan and spread it evenly. Bake for 45-55 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean. The top should be golden brown.

7. 7. Cool and glaze: Let the loaf cool in the pan for about 10-15 minutes before carefully removing it to a wire rack to cool completely. While it cools, prepare the optional lemon glaze by whisking together the powdered sugar and lemon juice until smooth. Drizzle the glaze over the cooled loaf just before serving.