It is often made by sautéeing or broiling liver and onions in schmaltz (i.e., rendered chicken fats); adding hard-boiled eggs, salt and pepper to the sautéed liver and onions, and grinding that mixture. However other methods and materials exist and so the exact process and ingredients may vary from chef to chef.
Chopped liver is a common menu item in Kosher delicatessens in Britain, Canada, and the U.S.A. Chopped liver is often served with rye bread as sandwiches.
The liver used is generally calves' liver, cow liver or chicken liver. Shortening or oil is often substituted for the schmaltz.
An alternate explanation for the etymology of the "What am I, chopped liver?" expression is that chopped liver was traditionally served as a side dish rather than a main course. The phrase, therefore may have originally meant to express a feeling of being overlooked, as a "side dish.