In a blender, add 1/4 cup skim milk, Greek yogurt, and then feta crumbles (in that order). Top with cilantro, lime juice, and serrano.
As the ingredients blend, pour in the remaining milk (and more to achieve desired thinness).
Save in a sealed container, like a Mason jar, and refrigerate for up to two weeks.
Recipe by Brandi Koskie. Photo by Dana Shultz.