In Harriet the Spy, Harriet is assigned the role of an onion. Her grade is in charge of representing dinner so they are all assigned different foods.
This role is actually very symbolic as there are many layers to an onion. What a person sees on the outside is not actually the part that is enjoyed. Harriet keeps learning things about people on her spy route - things you wouldn't know at first glance. People are also trying to learn and figure out Harriet, slowly peeling back her layers.