Rinse the preserved lemon to remove some of the saltiness.
Add all ingredients except milk to a blender or food processor. Blend until smooth. Thin with milk if needed.
Serve immediately or let sit for at least 30 minutes to intensify the flavors.
If you do not have preserved lemon you can substitute the zest of 1 fresh lemon and 1/4 teaspoon salt, plus use the lemon juice for thinning.