The piadina farcita mista is the loaded fold, the build that treats the round as a carrier for a full combination rather than a two-element pairing. A mista fold typically runs a cured meat, a melting cheese, and a leaf or a vegetable together, prosciutto or crudo with squacquerone and rocket being the common spine, the salt and fat of the meat, the cool tang of the cheese, and the bitterness of the green all held in one warm round. The defining fact is that the components are chosen to cover one another's gaps: the cheese is a soft bind with no edge of its own, the meat is salt and fat with no freshness, the leaf is bitterness with no body. None is a sandwich alone, and the round, plain and warm, is the neutral ground that lets the three read as one bite rather than three.
The craft is balancing a fuller filling without overloading a thin round. The piadina is cooked thin and folded hot off the testo so it stays supple under the weight, and the order matters: the soft cheese goes on first, smeared into the warm crumb so it both slackens against the heat and glues the rest in place, then the meat draped so it does not clump, then the leaves last so they stay dry and do not steam. The portion is judged so the fold can still close and be held in one hand, because the failure mode of a mista is greed, a round packed past the point where it can shut, the filling sliding out the open seam on the first bite. A good build keeps each element legible and the seam tight; a sloppy one buries the balance under too much of everything and weeps through the bread.
The variations are the single-element folds this combination is assembled from, each its own cleaner build. There is the squacquerone and rocket pair without the meat, the stracchino fold on its own, the grilled-vegetable version that swaps the salumi for char, and the crudo-forward build that leans on the cured meat rather than the cheese. Each of those is one decision pulled out of this mix and given room, and each deserves its own article rather than being crowded in here.