The crostino toscano gives up the second slice of bread and stakes everything on one warm spread. A small slice of Tuscan bread, typically the unsalted pane sciocco, is toasted or grilled firm and topped with a coarse, warm chicken-liver pâté, the fegatini, spread thick enough that the bread nearly disappears under it. There is no top slice and no closing the thing: it is open-face by design, eaten in one bite or two, the toast acting as a rigid base rather than a wrapper. That single-surface logic is the whole sandwich. Everything the prosciutto roll balances across two slices, the crostino has to balance on one, which is why the topping is strong and applied without restraint.
The craft is the base and the spread, and the two are engineered together. The slice is taken small and toasted dry so it stays rigid under a wet, rich load and does not buckle on the way to the mouth. The fegatini are chicken livers cooked down slowly with onion, a splash of wine or vin santo, capers, and anchovy until the mixture is soft enough to spread and assertive enough to carry a bite on its own. It goes on warm, so it just begins to slacken into the surface of the toast without soaking through. The salt of the anchovy and the sharp bite of the caper are doing the counter-work that a second ingredient would do in a closed sandwich; nothing else is needed and nothing else belongs.
The variations stay small and Tuscan, and each is its own preparation rather than a footnote here. The truffle version, restrained and pale rather than coarse and dark, is the Crostino al Tartufo; there are seasonal vegetable and cheese crostini that follow the same one-slice rule with a different topping. Each is a single slice under a single strong thing, and each deserves its own article rather than being crowded in here.