1. Prepare Your Muffin Tin: Start by preheating your oven to 375 F (190 C). Line a 12-cup standard muffin tin with paper liners or grease it thoroughly with non-stick spray. This prevents sticking and makes cleanup a breeze.

2. Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, salt, and poppy seeds. Make sure there are no lumps and everything is evenly distributed. This ensures the leavening agents are spread throughout the batter.

3. Mix Wet Ingredients: In a separate medium bowl, whisk the large egg until it’s slightly frothy. Then, add the milk, vegetable oil, unfed sourdough discard, lemon zest, and fresh lemon juice. Whisk until all the wet ingredients are well combined.

4. Combine Wet and Dry: Pour the wet ingredient mixture into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. It’s crucial not to overmix; a few lumps are perfectly fine. Overmixing can lead to tough muffins.

5. Fill Muffin Cups: Divide the batter evenly among the 12 prepared muffin cups. You can use an ice cream scoop for easy portioning. Fill each cup about two-thirds full.

6. Bake Until Golden: Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until the tops are golden brown and a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

7. Cool and Glaze (Optional): Once baked, remove the muffins from the oven and let them cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. While they cool, whisk together the powdered sugar and lemon juice for the glaze until smooth. Drizzle the glaze over the cooled muffins just before serving.