Preheat oven to 350°F. Coat a loaf pan with nonstick spray and set aside.
Melt butter and let cool while you mix the batter.
In a large bowl, mix together flour, sugar, poppy seeds, baking powder, baking soda, and salt.
In another bowl, whisk together sour cream, milk, egg, lemon juice, and lemon zest.
Create a well in the center of the dry ingredients and add wet ingredients. Stir to combine, just until no dry bits remain. If mixture is too thick, add a small amount of milk. Fold in melted butter.
Pour the batter into the prepared baking pan, spreading to the edges. Bake about 60 minutes, until a toothpick inserted into the center comes out clean. Let cool completely before slicing.