The panino con bagnet ross is led by its sauce, not its filling, and that sauce is the Piedmontese red one: bagnet ross, a cooked condiment of tomato, sweet peppers, and onion reduced slowly with vinegar and a little sugar until it is thick, glossy, and tart. In Piedmont it is one of the sauces that accompanies boiled meats, but as a sandwich it is the bagnet that sets the terms. It is sweet against sour, soft against the bread, with a concentrated vegetable depth that carries a bite on its own. Spread onto a roll, with cold sliced boiled beef or tongue laid in, it is the red sauce that the rest of the sandwich is arranged around rather than the meat.
The craft is the reduction and the bread that can take it. Bagnet ross has to be cooked down far enough that it holds as a thick paste rather than a wet relish, because a loose, watery sauce will bleed into the crumb and leave the bread sodden before the sandwich is finished. It goes on a crusted roll or a firm country slice with enough spine to carry a wet, acidic spread, and the meat it accompanies, usually a cut from the bollito, is sliced thin and cool so it stays supple against the warm tang of the sauce. The vinegar is the working element: it cuts the fat of boiled beef and lifts an otherwise heavy, soft filling, which is exactly the job it does on the plate transferred to bread. Quantity is controlled so the sauce frames the meat instead of drowning it.
The variations stay Piedmontese and each is its own preparation rather than a footnote here. There is the version on plain boiled beef alone, the one paired with tongue, the spicier reduction with more pepper and heat. Its direct counterpart, the green parsley sauce of bagnet vert, is the other half of the bollito table and a different idea entirely; that and the mixed boiled meats it dresses follow their own logic and deserve their own articles rather than being crowded in here.