01 - Place sour cream, mayonnaise, cilantro, lime juice, garlic, jalapeño (if using), cumin, salt, and pepper into a blender or food processor.
02 - Process on medium-high speed until completely smooth, scraping down the sides of the blender 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.
04 - Stop and taste the sauce, adding more 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.