The square sausage roll is the plainest reading of the Lorne slab: the sausage in a roll and nothing else. Lorne, or square sliced sausage, is a seasoned beef and pork mixture set in a loaf tin and sliced into flat slabs rather than piped into links, which gives it a fine, close, even texture and a shape that matches a roll exactly. The defining decision is the squareness itself. A slice of Lorne lays flat and covers the full face of the roll in one even band, edge to edge, where a link sausage leaves gaps and rolls out of the bread. This is the unadorned Scottish morning roll, the slab and the bread, and the egg-and-tattie-scone builds are all departures from this base.
The craft is in the griddle and the fat, because with nothing else in the roll the slab has to be right. It is cut to the width of the roll and cooked on a flat heat so both faces brown and the close-textured meat firms enough to hold its shape and not bend or slide when the roll is lifted. The fine grind means it cooks fast and renders quickly, so the timing is to colour the surface without drying the centre to a board. The roll is a soft floured morning roll, yielding enough to compress to the slab but sturdy enough to take the rendered fat without going to paste, and butter is the bridge that carries the salt of the sausage across to the wheat and seals the crumb. The one sauce, brown or red, is applied inside in a measured stripe so it cuts the fat without running through the bread or burying the seasoned meat the slab is there to deliver.
The variations are mostly about what is added on top of the slab. A fried egg brings a yolk that has to be managed against the flat sausage; a tattie scone turns it into a fuller Scottish stack; black pudding adds a second savoury slab. The wider regional and breakfast sausage shelf, the coarse peppery Cumberland coil, the sage-heavy Lincolnshire, the bacon roll under every local name, is each a different seasoning or cut on the same morning idea. Each deserves its own article rather than being crowded in here.