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

2. Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, poppy seeds, baking powder, baking soda, and salt. Make sure there are no lumps and everything is well combined. This ensures even distribution of leavening agents.

3. Mix Wet Ingredients: In a separate medium bowl, whisk together the active sourdough starter, whole milk, melted and cooled butter, large egg, lemon zest, and fresh lemon juice until smooth. The mixture should be uniform in color and texture.

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 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 and consistent portioning. Fill each cup about two-thirds full.

6. 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. The tops should spring back lightly when touched.

7. Cool and Glaze (Optional): Let the muffins cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. If making the glaze, whisk together powdered sugar and lemon juice until smooth. Drizzle over cooled muffins.