01 - Add sour cream, mayonnaise, cilantro, lime juice, garlic, jalapeño (if using), cumin, salt, and pepper to a blender or food processor.
02 - Process on high until completely smooth, pausing to scrape down the sides as needed to ensure an even consistency.
03 - Stream in water or milk, 1 tablespoon at a time, blending after each addition until the desired thickness is achieved.
04 - Taste the sauce and adjust salt, pepper, or lime juice as needed to balance the flavor.
05 - Transfer to an airtight jar or bowl, cover, and refrigerate for at least 30 minutes to allow the flavors to meld before serving.