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 long sides to easily lift the loaf out later.

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

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

4. 4. Combine Wet and Dry Mixtures: Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few streaks of flour are fine. Overmixing can lead to a tough loaf.

5. 5. Fold in Blueberries: Gently fold in the blueberries. If using fresh, they’ll incorporate easily. If using frozen, they might slightly tint the batter, which is perfectly normal. Distribute them evenly throughout the batter.

6. 6. Transfer to Loaf Pan and Bake: Pour the batter into your prepared loaf pan, spreading it evenly. Sprinkle the remaining 1 tablespoon of granulated sugar over the top of the batter. This will create a lovely slightly crisp, sugary crust.

7. 7. Bake Until Golden: 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 and the loaf should feel firm to the touch.

8. 8. Cool and Glaze: Let the loaf cool in the pan for about 10-15 minutes before carefully lifting it out onto a wire rack to cool completely. Once the loaf is cool, whisk together the powdered sugar and lemon juice in a small bowl until smooth. Drizzle the glaze generously over the top of the cooled loaf. Allow the glaze to set for a few minutes before slicing and serving.