Duck and orange is the afterlife of a roast, and it reads nothing like the hoisin-wrap version it is often confused with. This is roast duck, sliced from the bird, set on good bread with the one sauce that has long defined that meat: a sharp, slightly bitter orange dressing that cuts the fat the way a gravy did on the plate. The orange is not a garnish or a sweetener. It is the part that tells you which roast you are eating, the acid and the faint bitterness of the peel set deliberately against duck's richness so a cold cut does not read as one heavy, fatty note between two slices of bread.
The craft is in the meat and the moisture, and it is the craft of leftovers used well. Roast duck sliced thin and against the grain stays tender as a cold filling rather than going to rope, and the fat that makes a hot duck luxurious becomes the thing the sandwich has to balance once it is cold. The orange dressing supplies both the acid and the lubrication the cooled meat has lost, applied as a measured smear rather than a pool so it seasons without soaking the crumb. The bread is chosen with real structure, a bloomer or a sturdy white, because a heavy, slightly dry roast filling collapses soft white, and butter under the meat bridges it to the bread and stops the dressing going straight through.
The variations are a carvery in order, each turning on how the orange is handled against the bird. A marmalade-set version leans sweeter and stickier; a sauce built on the bitterer peel and a little of the roasting juices reads closer to the plated dish; the redcurrant and the watercress builds swap the counter while keeping the cold-roast logic intact. Those deserve their own articles rather than being crowded in here.