The defining move of this Friulian sandwich is that the cheese is not melted into the bread but turned into a brittle wafer first and then put inside it. Frico croccante is grated Montasio, the firm Alpine cow's-milk cheese of Friuli, spread in a thin disc on a hot pan and cooked until the fat renders, the proteins set, and the whole thing crisps into a lacy gold crackle that snaps like glass. Slid into a roll while it is still hot, the frico is the filling and the texture both: a sandwich whose centre is not soft and stretchy but shatteringly crisp, the cheese reading as crunch rather than melt, with the bread the only yielding thing in the bite.
The craft is in the cheese and the heat that turns it. Montasio is used because it has the fat and the protein structure to render and reset rather than just puddle: too young and it stays greasy and limp, too aged and it scorches before it sets, so a half-cured wheel is the one that crisps clean. The disc is kept thin so it sets brittle all the way through with no soft middle, and it is moved into the roll the moment it comes off the heat, because a frico that has cooled and sat picks up moisture from the air and from the bread and goes from crisp to leathery within minutes. The bread is plain and soft on purpose, a neutral, slightly yielding shell whose only job is to carry a fragile crisp wafer to the mouth without competing with it or softening it before the first bite. It is made to order and eaten at once.
The variations stay in Friuli and stay close to the cheese: the soft frico morbido bound with potato and onion, which is a different, tender thing folded into bread rather than a crisp one, and the version that scatters the hot crackle over a roll with a little cured meat. The potato frico in particular is its own dish. Each of those deserves its own article rather than being crowded in here.