Bacon and banana is the breakfast butty pushed to the point where salt meets sweet on purpose. It keeps the structure of the bacon sandwich, rendered rashers in soft bread, but swaps the usual sharp counter, the brown sauce or red sauce, for sliced banana, so the thing that cuts the bacon's salt is not acid but sugar. That substitution is the entire defining move and it is a deliberate one, not an accident of an empty cupboard: the sandwich is built around the contrast between cured, fatty, salty meat and soft, sweet fruit, the same logic that pairs bacon with maple on a plate, here folded into a roll.
The craft is heat, ripeness, and order. The bacon has to be properly crisp, because the banana brings no texture of its own and a soft rasher against soft fruit gives a sandwich with nothing to bite, so the meat is the only source of crunch and is treated accordingly. The banana is used just ripe rather than overripe: a soft, sugary banana turns to slick mush under the weight of the bread, while a firmer one holds in slices and keeps the sweet note discrete instead of smearing it across everything. The bacon goes in hot so its warmth softens the banana slightly and marries the two, and the bread is soft and plain because any assertive crust would fight a filling that is already an argument between two strong, opposite tastes. Butter is usually kept thin or left out, since the banana is doing the smoothing the butter would otherwise do. It is a fast griddle-and-go build, made and eaten warm, because it does not improve with waiting.
The variations stay on the sweet-salt axis: a drizzle of honey or syrup to push the sweetness further, peanut butter spread against the bread for a richer, heavier reading, the rashers swapped for a sweeter cure. Each is a distinct pairing and deserves its own article rather than being crowded in here.