I too am starting to learn/implement the onion architecture. From what I can tell, your original method (Customer Entity in the UI) is an acceptable practice. Look here for a graphical, linear, representation of the Onion Architecture:
http://jeffreypalermo.com/blog/the-onion-architecture-part-3/
You can see that the UI can interact directly with an underlying Object Model. Since your customer is not a direct DB entity, but instead loaded from an injected repository, this seems to fit the model of the Onion.
My assumption here is that the Customer Entity is really a composite of different, normalized, DB entities which are assembled via the injected repositories.
My understanding is that in principle, the interactions are done through abstractions, and the fact that the Customer Entity is an abstracted representation of the Customer Database, this is valid.
Please, correct me if any of the above assumptions/thoughts are incorrect.