Sausage and onion is the banger sandwich where the counter is sweet rather than sharp, and the fried onion is what defines it. The sausage holds constant: pork bangers fried until the casing has caught and the inside has set, split and laid in soft bread. The variable is the onion, and specifically what frying does to it. Onions taken slowly in the sausage fat until they have collapsed, gone brown at the edges, and turned jammy and sweet are not a crunchy garnish and not raw bite. They are a soft, dark, almost confiture layer whose entire job is to answer the fatty, herby, faintly sweet sausage with more sweetness and a melting texture rather than with acid. This is the structural choice that separates the sandwich from a sausage with brown sauce: where the sauce cuts the fat with vinegar, the onion goes the other way and leans into the richness, doubling down on soft and sweet, and the sandwich either earns that or becomes one cloying note.
The build works because the two soft elements are kept honest by the bread and the cut. The sausage is cooked all the way through so it bites cleanly rather than smearing into the onion, and it is split lengthways and laid flat so it sits stable and presents a browned face against the dark onion rather than rolling out of the fold. The onions are cooked all the way down rather than merely softened, because half-fried onions are watery and sharp and would weep into the bread and unbalance the whole thing; taken far enough they are dry, sticky, and concentrated. The bread is soft white or a soft roll, buttered to the edges so the onion's moisture and the sausage fat do not soak straight through, and it is sized to carry a generous, slightly wet filling without going to paste. Closed and pressed gently, it holds the sausage and the onion in one even layer rather than letting the loose onion slide out the back on the first bite.
The variations branch off the same fixed banger by changing what sits with it. A fried egg adds a yolk to manage; brown or red sauce swaps the sweet counter back for an acid one; the same filling answers to a bap, a butty, a barm or a cob across the regional bread words, and the leftover Sunday version beds the sausages on mash instead. Each of those deserves its own article rather than being crowded in here.