Roast chicken and mayonnaise is the bound default of the chicken set, the version most people mean when they say a chicken sandwich, and what defines it is the bind rather than the bird. Cold roast chicken on its own is mild and slightly dry; folded with mayonnaise it becomes a cohesive, moist, mildly rich filling that holds together and spreads evenly on bread. The mayonnaise is not a condiment laid on top, it is the medium the chicken sits in, supplying the fat and the moisture the cold meat lost off the bird and turning a loose handful of slices into something that behaves as one filling. This is the coronation-adjacent baseline: the same bound-chicken logic, before any spice or fruit is added.
The craft is in the ratio and the texture, and it is unforgiving because there is little else in the sandwich to distract from it. Too much mayonnaise and the filling slumps into a slick that soaks the bread; too little and it is dry and falls out the sides. The chicken is best torn or roughly chopped rather than sliced thin, so it holds the dressing instead of sliding, and it is seasoned through, because mild cold chicken and plain mayonnaise without salt and pepper read as nothing. A textural counter earns its place: a little chopped celery, spring onion, or cress breaks an otherwise uniformly soft filling on soft bread that would otherwise be one note. The bread is plain and the filling is spread evenly so each bite is the same, because an uneven bound-chicken sandwich is mostly bread at one end and a wet clump at the other.
The variations are small swaps on the same bound base. A grind of pepper and a squeeze of lemon brightens it; tarragon turns it herbal and closer to the tea tray; a sweeter dressing edges it toward coronation. Add crisp bacon and lettuce and it becomes the club-leaning build, while plain unbound roast chicken is the baseline version and stuffing the festive one. Each deserves its own article rather than being crowded in here.