Begin by cutting your avocado in half. Using the bottom edge of a sharp knife, hit the pit of the avocado, then twist and remove it. Discard the pit.
Remove the avocado peel.
If using fresh lime juice, squeeze 1 ½ - 2 limes until you have ¼ cup of juice.
There is no need to chop the cilantro as the food processor will do it for you. However, you may wish to remove some of the stems.
Add the avocado, mayonnaise or sour cream, lime juice, cilantro, and salt to a food processor or blender.
Secure the lid and blend or pulse for about 30 seconds, or until well combined.
The sauce should be rather thick. If you would like it thinner, blend in water 1 tablespoon at a time until you reach your desired consistency.
Taste and adjust the level of seasoning if necessary.
Use a rubber spatula to remove all of the sauce and place it into a bowl or an airtight container. Use immediately or store in the fridge!