1. 1. Prepare Your Baking Pan and Oven: 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 sides to easily lift the loaf out later.

2. 2. Combine Dry Ingredients: In a large mixing bowl, whisk together the 1 1/2 cups all-purpose flour, granulated sugar, baking powder, and salt. Make sure there are no lumps and the ingredients are evenly distributed.

3. 3. Mix Wet Ingredients: In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, egg, vanilla extract, and lemon zest until well combined. The mixture should be smooth.

4. 4. Combine Wet and Dry: Pour the wet ingredient mixture into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and help keep the loaf tender. Overmixing can lead to a tough loaf.

5. 5. Add Blueberries: In a small bowl, toss the fresh 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. 6. Bake the Loaf: Pour the batter into your prepared loaf pan and spread it evenly. Bake for 50-60 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, whisk together the powdered sugar and lemon juice for the optional glaze. Once the loaf is fully cooled, drizzle the glaze over the top.