1. Prepare your muffin tin and preheat the oven: Start by preheating your oven to 400 F (200 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with butter or non-stick spray. This ensures your muffins won’t stick and makes for easy cleanup.

2. Combine the 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 all ingredients are evenly distributed. This step is crucial for even rising.

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

4. Gently combine wet and dry mixtures: Pour the wet ingredients into the dry ingredients. Stir with a spatula or wooden spoon just until barely combined. The batter will be thick and lumpy, and that’s exactly what you want! Overmixing leads to tough muffins, so stop as soon as you no longer see streaks of dry flour.

5. Fold in the blueberries: Carefully add the fresh or frozen blueberries to the batter. Gently fold them in with your spatula, distributing them evenly without crushing them. If using frozen blueberries, do not thaw them first.

6. Fill the muffin cups: Divide the batter evenly among the 12 prepared muffin cups. Each cup should be about two-thirds full.

7. Add a sugary topping: Sprinkle the remaining 1 tablespoon of granulated sugar over the tops of the filled muffin cups. This creates a lovely, slightly crisp crust.

8. Bake until golden: Bake for 20-25 minutes, or until the muffins are golden brown and a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

9. Cool before serving: Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. This prevents them from becoming soggy on the bottom.