It’s become a trope in American movies: A concerned friend will turn up on the doorstep of the grieving or recently heartbroken protagonist with a homemade casserole. Often you won’t see the food being cooked or eaten. It is enough just to see the foil-covered rectangular glass or ceramic baking dish, a universally recognizable symbol (at least in the US) of compassion and community.

The history of how casseroles became the definitive American comfort food is also the history of the country in the first half of the 20th century. The dish—which was originally defined as a combination of vegetables, starch and meat—became a useful way of stretching limited resources during the privations of two world wars and the Great Depression. But it is also a story of American technical innov...