Origin of "chopped liver"

what is the origin of "chopped liver"?

Literally, chopped liver is a traditional Jewish dish, cooked chicken livers chopped or ground and seasoned, sometimes used as a sandwich filling. "What am I, chopped liver?" means "Don't dismiss me as if I were insignificant." I recently heard an explanation of this saying. Unfortunately, I don't remember where. It went like this: Chopped liver is always served as a side dish, never as a main dish. It therefore makes a good metaphor for someone who's being treated as unimportant or dispensable.