The Chicago-style egg roll is the rare American sandwich that arrives as a sealed deep-fried tube and is sometimes then put inside a bun, doubling the carbohydrate and committing fully to the format. Strip away the takeout-counter context and what defines it is the filling: ground beef rather than pork, bean sprouts for water and crunch, and a wrapper fried hard enough to shatter. It is a stuffed-pocket sandwich whose container is its own crisp shell, and the bun, when it appears, is a second carrier wrapped around the first.
The craft is in the seal and the dip. The wrapper has to be rolled tight and fried at a temperature high enough to set a glassy, blistered crust before the ground-beef-and-sprout filling steams it soft from inside, which is the same closure problem every fried hand pie has to solve. The filling is cooked before it goes in, because the fry time is set by the wrapper, not the center. What makes it Chicago is the finish: a swipe of fierce yellow hot mustard for heat and a sweet red sauce for the cooling counterweight, applied so the two opposing condiments hit in the same bite. Where it is served in a bun, the soft bread does a third job, absorbing grease and giving the hand something that will not burn it, and turning a snack into something closer to a sandwich proper. The structural risk is timing: a fried roll that sits goes limp, and the mustard's bite fades, so it is built to be eaten fast and hot off the counter.
This sits at the edge of what counts as a sandwich, and the catalog is full of these boundary cases: the egg roll proper, the fried hand pie, the regional pocket that builds its own bread around the filling. Each of those traditions deserves its own article rather than being crowded in here.