The Glamorgan sausage sandwich is a meat sandwich with no meat in it, and that contradiction is the entire point. The Glamorgan sausage is a Welsh preparation of grated Caerphilly, leeks, breadcrumbs, mustard, and egg, bound into a soft cylinder, rolled in more crumbs, and fried until the outside is crisp and the inside is molten and savoury. It is shaped like a sausage, browned like a sausage, and slotted into a roll like a sausage, but what it delivers is the sharp, lactic tang of a young Welsh cheese cut with the green sweetness of leek. The sandwich works because the sausage already behaves like a cooked filling: it holds together when sliced, it browns under heat, and it brings its own seasoning, so the bread's only job is to carry it and add a soft contrast.
The craft is in the bind and the crust, because both are load-bearing. The mixture has to hold enough egg and crumb to survive frying without slumping into the pan, but not so much that it stops tasting of cheese and leek, and the calibration of that ratio is what separates a good Glamorgan from a dense one. The crust is structural as much as textural: a properly fried crumb shell keeps the soft centre contained inside the roll instead of letting it smear, which is the same problem a croquette solves. A soft floured roll suits it because the sausage carries the flavour and the heat, and a bread with real chew would only fight a filling that has no resistance of its own. Mustard or a sharp chutney is the usual counter, applied as a stripe rather than a flood, there to cut the richness of fried cheese rather than to add a competing note.
The Glamorgan sits on the Welsh shelf alongside the island's other place-named savouries, and within the meat-free corner of the British sandwich it is the one that most convincingly impersonates a banger. Its near relatives are codified by their counter rather than their core: the Glamorgan with leek-heavy mustard, the version pushed toward a breakfast roll with a fried egg, the Caerphilly cheese sandwich that uses the same cheese without the frying. Those deserve their own articles rather than being crowded in here.