The Forfar bridie is not a sandwich but it solves the sandwich's problem by baking its own container, and the horseshoe shape is the tell. A round of pastry is rolled, loaded along one half with seasoned minced beef and onion, folded over into a half-moon, and crimped along the curved edge into a thick rope seam. There is no potato and no swede in it, which is the line that separates a bridie from a Cornish pasty: the filling is leaner, beefier, and noticeably peppery, and the pastry has to carry it without the moisture a root-vegetable filling would lend. The defining fact is that the bread is engineered around the cargo, sealed into a pocket rather than laid in slices, so the bridie belongs to the closed-pastry family rather than the open or stacked one.
The craft is the pastry and the closure, because everything else follows from those. The dough is a sturdy one with enough structure to hold a wet meat filling through a full bake and survive a coat pocket on the way home, not a delicate flaky shell that would shatter and vent. The beef goes in raw and well seasoned, because the bake time is set by the crust browning rather than the meat cooking, and because the crimped crust is the only wrapper the filling gets, so the pepper and salt have to carry it on their own without a sauce. The rope seam along the curved edge is the structural heart of the thing: pressed firm and unbroken it holds the steam in and cooks the mince gently inside its own shell, and it doubles as the spine you grip while the rest is eaten. A split crimp dries the filling and the bridie fails on the walk.
The variations stay inside the sealed, crimped frame. A plain beef bridie and an onion-heavy one shift the filling without touching the structure; a version slid into a soft roll doubles the carbohydrate and turns the parcel into a fuller meal; the Scotch pie and the wider pasty family are the same engineered-container instinct met in other Scottish and regional forms. Each is its own thing and deserves its own article rather than being crowded in here.