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, pausing to scrape down the sides of the blender or food processor as needed.
03 - Stream in water or milk, 1 tablespoon at a time, blending after each addition, until the sauce reaches your desired 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.