1. 1. Prepare Your Baking Pan and Oven: Preheat your oven to 350 F (175 C). Grease and flour a 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the long sides to easily lift the loaf out later.

2. 2. Combine Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.

3. 3. Cream Butter and Sugar: In a large bowl, using an electric mixer (handheld or stand mixer), cream together the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes. The mixture should be pale yellow and airy.

4. 4. Add Wet Ingredients (Except Sourdough and Milk): Beat in the eggs one at a time, mixing well after each addition until fully incorporated. Scrape down the sides of the bowl as needed to ensure everything is well combined.

5. 5. Alternate Wet and Dry Mixtures: Gradually add the dry ingredient mixture to the wet ingredients, alternating with the sourdough discard and milk. Begin and end with the dry ingredients. Mix on low speed until just combined, being careful not to overmix. Overmixing can lead to a tough loaf.

6. 6. Fold in Flavorings: Gently fold in the lemon zest, fresh lemon juice, and poppy seeds until evenly distributed throughout the batter.

7. 7. Bake the Loaf: Pour the batter into your prepared loaf pan and spread evenly. Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean.

8. 8. Cool and Glaze: Let the loaf cool in the pan for 10-15 minutes before carefully removing it to a wire rack to cool completely. While the loaf cools, prepare the glaze by whisking together the powdered sugar and 2-3 tablespoons of fresh lemon juice until smooth. Once the loaf is completely cool, drizzle the glaze over the top. Allow the glaze to set before slicing and serving.