The Sandwich Potjevleesch is a slice of jellied terrine laid into bread. Potjevleesch, "little pot of meat" in West Flemish, is the Flanders potted-meat dish: rabbit, chicken, pork, and veal cooked separately, then packed into a pot and set in a clear, lightly acidic aspic until the whole thing firms into a sliceable block of pale meat suspended in jelly. To make the sandwich you cut a cold slab straight from the pot, lay it on a split crusted loaf, and add almost nothing. The jelly is the sauce, already built into the meat. The region is Flanders.
The craft is the behavior of the aspic. The jelly is what makes potjevleesch sliceable and what carries its faint vinegar lift, so it has to stay cold: warm it and the block slumps, the jelly weeps, and the bread takes on water it cannot hold. The successful sandwich is therefore a cold one, the terrine cut thick enough to keep its shape against the crumb, the bread crusty enough to give the soft, gelatin-bound interior something to brace on. The four meats read differently across one slice, the rabbit and chicken lean and mild, the pork and veal rounder, the jelly tying them with a clean sourness that does most of the work a condiment would otherwise do. That acidity is the reason the sandwich does not need much: a smear of mustard along one face sharpens it further, a cornichon adds a second sour note, and anything richer fights the jelly rather than helping it. It pairs the way the dish itself does in Flanders, with frites or with a glass of cool blonde beer, eaten slowly because the cold slab does not rush.
Variations are mostly a question of which meats dominate the pot and how sharp the jelly is set, more rabbit in one kitchen, a firmer or softer aspic in the next, a leaf of frisée added for a little structure and bitterness. The Sandwich Potjevleesch belongs with the regional dishes the catalog groups under Plat-en-Sandwich, the Flanders and northern preparations that move from the pot to the loaf. Its specific contribution is a jellied multi-meat terrine that must be eaten cold, a sandwich whose sauce is the set aspic and whose discipline is to keep it that way.