Without more context, my guess is that he's saying to metaphorically "look deeper". The "surface" is what is obvious about a situation, "peeling the onion" would reveal a layer below the surface, the true meaning of a situation. On the surface, The Wizard of Oz is a story about a girl's adventure in a strange land, but if you peel the onion, you could see that there is a hidden condemnation of America's economic policies.
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