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 high until completely smooth, stopping to scrape down the sides as needed to ensure even blending.
03 - Add water or milk one tablespoon at a time, blending briefly 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.