01 - Add sour cream, mayonnaise, cilantro leaves, lime juice, garlic, jalapeño (if using), cumin, salt, and pepper to a blender or food processor.
02 - Process on medium speed until the mixture is completely smooth, scraping 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, stopping once the sauce reaches your preferred thickness.
04 - Taste the sauce and adjust salt, pepper, or lime juice as needed to balance the flavor.
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.