1. Preheat your oven to 375 F (190 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 bread out later.

2. In a large mixing bowl, whisk together the sourdough discard, 1/2 cup granulated sugar, melted butter, egg, and vanilla extract until the mixture is smooth and well combined.

3. In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. If using lemon zest, add it to the dry ingredients here and toss to distribute.

4. Gradually add the dry ingredient mixture to the wet ingredient mixture, stirring gently with a spatula or wooden spoon until just combined. Do not overmix.

5. Gently fold in the fresh or frozen blueberries. If using frozen, do not thaw them first. (Optional: toss blueberries lightly with a tablespoon of flour before adding to prevent sinking).

6. Pour the batter into your prepared loaf pan and spread it evenly. Sprinkle the remaining 2 tablespoons of granulated sugar over the top of the batter.

7. Bake for 30-35 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 edges should be pulling away slightly from the pan.

8. Once baked, remove the loaf pan from the oven and let it cool in the pan for about 10-15 minutes on a wire rack. Then, use the parchment paper overhang or a knife to carefully remove the bread from the pan and transfer it directly to the wire rack to cool completely before slicing and serving.