1. Preheat your oven to 350 F (175 C). 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 mixing bowl, whisk together the 1 1/2 cups all-purpose flour, granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and everything is well combined.

3. In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, egg, vanilla extract, and lemon zest until smooth. The discard doesn’t need to be perfectly incorporated, a few small lumps are fine.

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 streaks of flour are perfectly acceptable. Overmixing can lead to a tough loaf.

5. In a small bowl, toss the blueberries with the remaining 2 tablespoons of all-purpose flour. This step helps prevent the blueberries from sinking to the bottom of the loaf during baking. Gently fold the floured blueberries into the batter until evenly distributed.

6. Pour the batter into your prepared loaf pan and spread it evenly. Place the pan in the preheated oven and bake for 45-55 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 making the glaze, whisk together the powdered sugar and lemon juice until smooth. Drizzle over the cooled loaf before slicing and serving.