The defining ingredient here is an absence: salt, deliberately left out of the bread. Tuscan bread, pane sciocco, is made without salt, which gives it a flat, almost sweet, faintly sour crumb that tastes wrong eaten on its own and is meant to. Set against salumi, the salty cured meats of the region, that blandness stops being a flaw and becomes the entire point. Finocchiona shot through with fennel, prosciutto toscano cured hard and assertive, coarse Tuscan salame: each is intensely seasoned, and the unsalted loaf is the calm, neutral ground that lets the meat be as loud as it is without the whole sandwich tipping into pure salt.
The craft is in trusting the contrast and not correcting it. The bread is cut thick, with a chewy crumb and a firm crust, because a Tuscan loaf has the structure to carry a fatty, heavily seasoned filling and because its plainness is doing real balancing work that a flavoured bread would undo. The salume is sliced to suit its character: a fatty prosciutto cut paper-thin and laid in loose folds so air gets through it, a firm salame cut thicker to keep its bite, finocchiona sliced generously because its fennel is the dominant note. No butter, no oil, no sauce, because every one of those would either fight the meat or paper over the very contrast the unsalted bread exists to provide. The whole composition is two elements and the gap between them.
The variations stay Tuscan and stay built on the same plain loaf: the bread with finocchiona alone, with prosciutto toscano, with a coarse mixed plate of regional salumi, sometimes a slick of lardo di Colonnata sliced to translucence so the fat melts against the crumb. The grilled, garlic-rubbed, oiled version of the same bread is fettunta. Each of those is the same unsalted Tuscan loaf met by one cured thing, and each deserves its own article rather than being crowded in here.