The Cornish Yarg sandwich is defined by its rind, and the rind is a leaf. Yarg is a semi-hard Cornish cow's-milk cheese wrapped, while young, in nettle leaves that are pressed onto the surface and left to develop a thin, edible, mottled grey-green coat as the cheese matures. Inside that wrapper is a fresh, mild, faintly lemony paste with a crumble near the rind and a creamier centre. The defining fact of this sandwich is that the leaf is part of what you eat: the nettle rind carries a soft mushroomy, mineral, earthy note that the pale interior on its own does not, and a slice taken with the rind on tastes of two different things at once where most cheese sandwiches taste of one.
The craft is the cut and the restraint. Yarg is a delicate, lactic cheese rather than a loud aged one, so it is sliced rather than crumbled and laid thick enough to have presence without drying the mouth, and crucially it is cut so that every slice carries a band of the nettle rind, because the rind is the point of difference and a rindless slice is just a mild young cheese. The bread is soft and plain and the butter underneath is the bridge, because anything assertive, a sharp pickle or a strong loaf, would flatten the subtle earthy note the leaf provides rather than support it. Where the Cheddar sandwich is built to stand up to a pickle, the Yarg sandwich is built to be left alone, with at most a mild salad leaf or a thin smear of chutney that defers to the cheese instead of competing with it.
The variations stay close, because the rind is doing the work. Yarg with a few peppery salad leaves echoes the green note of the nettle wrap and keeps everything gentle. The garlic-and-wild-herb wrapped version of the same cheese swaps the leaf and so swaps the rind flavour while keeping the structure. The wider British cheese sandwich, Cheddar, Wensleydale, the blues, is a louder family that earns a pickle. Each of those is its own sandwich and deserves its own article rather than being crowded in here.