One theory traces it to Italian-American weddings in 1920s Chicago, where beef sliced paper-thin and simmered in gravy could stretch to feed a crowd.
Thin-shaved roast beef soaked in seasoned gravy, piled into a long French roll and topped with sweet peppers or hot giardiniera; ordered at the counter, often 'dipped' so the whole sandwich is dunked in the jus.
Philly Cheesesteak