1. 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 long sides to easily lift the loaf out later.

2. In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and everything is evenly distributed.

3. In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, egg, lemon zest, and fresh lemon juice until well combined. The mixture should look smooth and slightly creamy.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few small lumps are perfectly fine. Overmixing can lead to a tough loaf.

5. Gently fold in the fresh or frozen blueberries. Distribute them evenly throughout the batter without crushing them.

6. Pour the batter into your prepared loaf pan and spread it evenly. Place the pan in the preheated oven and bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The top should be golden brown.

7. Once baked, remove the loaf from the oven and let it cool in the pan for about 10-15 minutes. Then, carefully lift it out using the parchment paper overhang (if used) or invert it onto a wire rack to cool completely. If adding the optional glaze, whisk together the powdered sugar and lemon juice until smooth, then drizzle over the cooled loaf.