01 - Place the sour cream, mayonnaise, cilantro leaves, lime juice, garlic clove, seeded jalapeño (if using), ground cumin, sea salt, and black pepper into a blender or food processor.
02 - Process on high until the mixture is completely smooth and uniformly green, pausing to scrape down the sides of the bowl as needed.
03 - Stream in water or milk one tablespoon at a time while blending, stopping once the sauce reaches your preferred thickness.
04 - Taste the sauce and adjust salt, pepper, or lime juice as desired to balance the flavors.
05 - Transfer the sauce to an airtight jar or bowl, cover, and refrigerate for at least 30 minutes to allow the flavors to meld before serving.