The Arbroath smokie sandwich is a case where the bread is instructed to do almost nothing. The smokie is a haddock hot-smoked over hardwood until the flesh is cooked through, coppery, and dense with smoke, a fish so fully finished and so strongly flavoured that any assertive bread or sauce would be an argument it does not need. The sandwich is therefore built as a plain frame around a single loud component: soft bread, butter, perhaps a squeeze of lemon, and the warm flaked fish. The decision that defines it is subtractive. The skill is knowing how little to add to a fish that is already complete.
The craft is in handling the fish rather than constructing the sandwich. A smokie is warmed gently and the flesh lifted off the bone in large flakes, kept loose so it stays moist and does not pack into a dense paste between the slices. Because the fish carries oil, smoke, and salt on its own, the butter is there for lubrication and to bridge the fish to the bread, not for flavour, and the bread is plain white or a soft roll precisely so it disappears behind the haddock. This is a sandwich that barely travels: it belongs to the Angus coast where the fish is made, eaten close to source and close to warm, and that locality is the whole identity rather than an inconvenience.
The variations stay tied to the same regional fish. A poached egg turned through the flakes makes it a fuller plate folded into bread; soft cheese or a horseradish cream cuts the smoke for those who want it cut; the cold-smoked Finnan haddie is a quieter relative of the same coast. Those are distinct enough to deserve their own articles rather than being crowded in here.