The panino con mondeghili is a Milanese frugality dish that ended up in bread. Mondeghili are fried polpette built from leftover boiled meat, the bollito, minced and bound with mortadella, soaked bread, Parmigiano, egg, and a little garlic and parsley, then shaped flat, breaded, and fried until the crust is dark and the inside stays soft. The defining fact is that nothing in them is cooked from raw for the dish: it is a way of turning yesterday's meat into something new and better. Piled hot into a plain roll, the mondeghili are the entire statement, and the bread is a deliberate blank that exists to make a fried, fork-and-plate thing portable.
The craft happened at the mixing bowl and the fryer, before any bread is involved. The boiled meat is minced fine and the mortadella worked through it for fat and a faint spice, the soaked bread and Parmigiano giving the mix the tenderness that keeps a fried patty from going dense. They are shaped flattish rather than round so they sit in a split roll without rolling out, breaded so the exterior shatters against the soft interior, and fried so they are eaten hot, when the contrast is sharpest and the fat reads cleanest. The roll is plain and sturdy and is not sauced, because the polpette already carry salt, fat, and cheese; a dressing would only argue with a thing that is already complete.
The variations are narrow and Milanese, mostly about what the mondeghili sit in and beside. There is the classic on a soft roll, the version with a little mostarda or a sharp pickle alongside for a cutting note, and the related leftover-and-stall plates of the same Lombard thrift tradition. Each is the same fried recovered-meat polpetta given a handle, and those deserve their own articles rather than being crowded in here.