Ham and mustard is also the default pre-made sandwich, and read from that side it is a different object from the one built fresh at home. This is the ham and mustard of the chiller cabinet and the meal deal: a fixed pack assembled hours before it is eaten, sliced on the diagonal, sealed in a wedge, and engineered to survive the gap between a factory and a lunch break. The mustard here is doing the same job, cutting the salty sweetness of cooked ham with a jolt of heat, but the constraint is no longer freshness. It is shelf life. The whole build is shaped by the requirement that it still read as a sharp ham sandwich after a day in a cold cabinet rather than minutes after assembly.
The craft, in the made-round and the packed form, is moisture and balance over time. The mustard is spread thin and even across the bread because an uneven smear in a sandwich that nobody is watching becomes a hot clump at one end and bare bread at the other. Butter to the edges is structural waterproofing, not flavour: it seals the crumb so the bread does not go grey and damp before the pack is opened. The ham is laid in an even, overlapping layer so every diagonal bite carries the same ratio of meat to heat, because the eater is not adjusting it as they go. The bread is soft and plain and cut on the slant so the pack holds two neat triangles that survive handling. It is a sandwich designed to be made once and judged later.
The variations are the rest of the cabinet shelf. The same ham and mustard built fresh and eaten at once is the leaner, sharper parent, the version made by hand rather than to a spec. A wholegrain mustard pack softens the heat into something seedier; a ham, cheese, and mustard build adds fat the mustard then cuts; a sub or baton format trades the triangle for a roll. Each of those is its own sandwich and deserves its own article rather than being crowded in here.