Adding apple to a black pudding sandwich is not a garnish decision; it is a correction. Black pudding fried on its own is dark, dense, and iron-heavy, a filling that arrives at one strong note and holds it. A few slices of apple fried in the same pan answer that note directly: the heat softens them, draws out sugar, and concentrates a tartness that cuts straight through the mineral weight of the blood. The apple is the variable that makes the constant edible in quantity. Take it away and you have a good sandwich that is a little relentless; put it in and the relentlessness becomes contrast.
The craft is timing the two elements to the same pan and the same heat. The pudding is sliced and fried hard so its cut faces crisp while the inside stays soft and crumbly; the apple goes in to catch the rendered fat and caramelise at the edges without collapsing to sauce. Both have to be hot when the sandwich is built, because a warm apple against a hot pudding reads as sweet relief while a cold one against a cooling slice reads as two separate things that happen to share bread. The bread is soft and plain, a floured roll or buttered white, there to soak a little fat and hold a crumbly, slightly wet filling without arguing with it. Butter underneath bridges the spiced iron of the pudding to the wheat and keeps the apple's moisture off the crumb.
This is one answer to what black pudding wants beside it, and the others are answers to the same question. A fried egg binds the crumble with yolk instead of cutting it with acid; bacon doubles down on salt rather than relieving it; brown sauce and a flat mushroom pull the whole thing toward the full breakfast. Those deserve their own articles rather than being crowded in here.