The Lincolnshire sausage sandwich is defined by a single herb. A Lincolnshire is a coarse-cut pork sausage seasoned heavily with sage, so much that the cut surface is flecked green and the smell is unmistakable before the meat is anywhere near bread. That sage, against a rough open texture rather than a fine paste, is the whole identity. The sandwich exists to carry the herb forward, not to soften it. Where a plain breakfast banger wants a sauce to give it interest, a Lincolnshire arrives already loud, and the build is arranged to frame a strongly seasoned, coarsely chopped sausage rather than to add character it does not already have.
The craft is in the cut and the fat. The links are split or butterflied and pressed flat so they sit low and do not roll out of the bread, and they are browned hard so the coarse meat firms and the sage develops into a savoury crust rather than steaming pale and one-note. The roll or sliced bread is soft enough to yield to a substantial filling but sturdy enough to take the rendered fat without going to paste, and butter bridges the salt of the sausage to the wheat while sealing the crumb against the grease. The one condiment is a sharp counter applied inside in a measured stripe, brown sauce or a sharp mustard, judged so it cuts the richness without flooding the bread or burying the sage the sausage is prized for. Restraint is the rule, because the sausage has already done the seasoning and the build only has to keep out of its way.
The variations are mostly a question of what is set against the sage. A fried egg adds a yolk that has to be managed against the herb. Caramelised onion brings a sweet counter to the savoury. Apple sauce, borrowed from the pork tradition, leans the sandwich sweet. The other regional sausage sandwiches, the coarse peppery Cumberland sold as a coil, the cheese-and-leek Glamorgan behaving like meat, the square Lorne sliced flat, are each a different seasoning on the same idea and deserve their own articles rather than being crowded in here.