01 - Place the sour cream, mayonnaise, cilantro leaves, lime juice, garlic clove, jalapeño (if using), ground cumin, sea salt, and black pepper into a blender or food processor.
02 - Process on high until completely smooth, pausing to scrape down the sides of the bowl as needed to ensure even blending.
03 - Stream in water or milk one tablespoon at a time while blending, until the sauce reaches your preferred thickness — thinner for drizzling, thicker for dipping.
04 - Stop and taste the sauce, adding more salt, pepper, or lime juice as needed 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.