Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large mixing bowl, whisk together the active sourdough starter and granulated sugar until combined.
Add the melted butter, eggs, whole milk, lemon zest, and lemon juice to the bowl. Whisk until smooth and well incorporated.
In a separate bowl, sift together the all-purpose flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, stirring gently with a spatula until just combined. Do not overmix.
Fold in the poppy seeds evenly throughout the batter.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 18 to 20 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and allow them to cool in the pan for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.