The haggis sandwich is defined by the fact that the filling arrives already finished, and finished in a way that asks the bread for almost nothing. Haggis is a Scottish preparation of sheep's heart, liver, and lung minced with oatmeal, suet, onion, and a heavy hand of pepper, cooked in a casing until it is a warm, crumbly, savoury mass. By the time it reaches a roll it is fully cooked, fully seasoned, and structurally loose: it does not slice into clean layers so much as break into a peppery, oat-bound crumble. The sandwich is the recognition that this filling is self-sufficient. It carries its own salt, its own fat, its own heat, and its own pepper, so the build is not about adding flavour but about holding a warm crumble together long enough to eat it in the hand.
The craft is heat and containment. Haggis is at its best warm, when the suet is soft and the oatmeal is loose rather than set, so it goes into the roll hot and is eaten before it tightens. The roll's only real job is structural: a soft floured morning roll yields to the crumble and soaks a little of its fat without disintegrating, and its plainness is deliberate, because a bread with assertive flavour or real chew would fight a filling that is already loud and offers no resistance of its own. The pepper in the haggis is the seasoning, so the sandwich classically carries no sauce at all; where one appears it is restrained, a thin stripe rather than a flood, there to add moisture rather than to compete. The whole construction is an argument that some fillings need only to be kept warm and contained, and haggis is the clearest case of it.
The haggis sandwich is the base of a small Scottish family, the one stripped to haggis and a roll, and its near relatives are codified by the single thing they add. Haggis and egg binds the crumble with a soft yolk for a breakfast; the haggis, neeps and tatties roll reassembles the full Burns supper inside the bread; a stripe of whisky sauce or brown sauce pushes it toward a wetter, richer reading. Those deserve their own articles rather than being crowded in here.