The black pudding sandwich is the rare sandwich whose filling is already a finished, seasoned thing, and the bread's only job is to stay out of its way. Black pudding is blood set with oatmeal and fat and heavily spiced, a dark, dense, mineral-tasting sausage with a crumbly rather than springy bite. Slice it, fry the slices so the cut faces crisp and the centre stays soft, and you have a filling that needs no sauce because it is its own seasoning and no acid because, on its own terms, it is balanced already. That self-sufficiency is the defining fact: this is a sandwich that subtracts rather than builds, Lancashire's blunt confidence in one ingredient.
The craft is the fry and the bread. The slices have to hit the pan hot enough to set a crisp face quickly, because a black pudding slice that steams instead of frying turns to a damp, sandy paste and the whole sandwich goes with it. The bread is soft and plain, usually a floured roll or buttered white, chosen to soak a little of the rendered fat and frame the crumbly texture rather than fight it. Butter is structural, the bridge between an iron-heavy, slightly dry filling and the wheat. It is eaten hot, straight off the pan, because black pudding cools fast and a cold slice loses the crisp edge that was the entire reason to fry it.
The pudding is the constant; the additions are a short, well known argument about what it wants beside it. Apple brings a sweet, acid cut that lifts the iron; a fried egg adds a yolk that binds the crumble and softens the spice; brown sauce, bacon, and a flat mushroom each pull it toward the full breakfast. Those deserve their own articles rather than being crowded in here.