The panino con frittata di pasta is carbohydrate on carbohydrate, and it is built that way on purpose. The frittata di pasta, a Neapolitan thing, is leftover dressed pasta, often spaghetti, bound with egg and a little cheese and fried in a pan until both faces form a hard crust and the inside compresses into a dense, sliceable cake. Putting a wedge of that between bread is not an accident of frugality so much as its logical end: the fried pasta cake is already a self-contained block, and bread is simply the handle that lets you eat it walking, on a train, or on the rocks at the shore. The defining fact is that nothing here is light, and the sandwich does not pretend otherwise.
The craft is in the crust and the bind. The pasta is mixed through with beaten egg so it sets into one mass rather than a tangle, and it is fried slowly enough that a real crust forms top and bottom before the centre is cooked, because that shell is what holds the wedge together once it is cut and what gives the bite its contrast against the soft interior. Day-old pasta works better than fresh: it is drier, holds its shape in the pan, and crisps instead of slumping. It is cut to the size of the roll and eaten at room temperature, when the crust is firmest and the egg has set hardest. The bread is deliberately plain and unfussy, a soft roll or a piece of ciabatta, because the frittata di pasta is the entire event and a strong loaf would only get in its way.
The variations stay Neapolitan and few, each its own thing rather than a line here: the version studded with salame and provola through the pasta, the plain spaghetti-and-egg one of strict cucina povera, and the broader habit of folding a set egg into bread, which is the Panino con Frittata and a separate piece. Each is a fried starch given a loaf, and each deserves its own article rather than being crowded in here.