Preheat the oven to 400 degrees. Grease a loaf pan or 4 mini loaf pans and set aside.
In a large bowl cream together the butter, sugar, and brown sugar. Add the eggs and mix well.
Stir in the sourdough discard and the milk and mix until smooth.
In a medium bowl combine the flour, baking powder, and salt. Add to the bowl with the wet ingredients and mix until a thick batter forms.
Add the zucchini, lemon juice, and poppyseeds to the bowl and mix until the batter is well combined.
Pour into the prepared loaf pan or mini loaf pans.
Bake the loaf pan for 50 minutes or the mini loaf pans for 25-30 minutes. The bread is ready is you stick a tootpick in the center and it comes out clean.
Remove from oven and cool completely on a wire rack before removing from pan.