Place avocado, mayonnaise, milk, lime juice, dill, cilantro, salt, pepper, garlic, and onion powder into a blender or food processor.
Puree until completely smooth. Taste and add more dill, salt, or pepper, to taste.
If desired, thin out the mixture with additional milk, pulsing to combine until you reach a pourable texture.
TO store, transfer to an airtight container. Press plastic wrap onto the surface, then seal with the lid. The avocado ranch will keep in the refrigerator 3-4 days.