Chicken and tarragon is the chicken-mayonnaise tea sandwich defined by an aniseed herb, and the herb has to be in the bind rather than on it. Poached chicken is chopped and held in mayonnaise, as in every sandwich in this family, but tarragon, with its distinct soft liquorice and anise note, is chopped fine and worked through the dressing so it perfumes the whole filling evenly. Tarragon and chicken is one of the oldest reliable pairings in a cook's repertoire precisely because the herb's sweetness flatters mild poached meat without overpowering it, and putting it through the bind rather than scattering it on top is what lets that pairing carry across every bite. Strip the tarragon out and the sandwich collapses back into plain chicken mayonnaise.
The craft is dosing a strong herb so it flavours without dominating. Tarragon climbs quickly and finishes long, so it is used with restraint and chopped finely enough to distribute through the mayonnaise rather than arriving in green clumps that taste sharply of anise in one bite and of nothing in the next. The chicken is poached gently and kept tender, since tarragon is an aromatic, not a tenderiser, and a dry, stringy filling under a fragrant dressing reads as dry first and fragrant second. The mayonnaise is kept thick so the chopped meat holds in a clean layer rather than sliding, and the bread is soft, plain, and buttered to the edges to seal the crumb against a slightly wet filling and to keep an assertive crust from arguing with a delicate flavour. Cut into crustless fingers, it is built to taste composed rather than abundant.
The variations are the rest of its own family, each turning on a different single aromatic. Chicken and lemon swaps the aniseed herb for bright acid in the same bind; chicken and watercress sets a peppery green against the meat; coronation chicken takes the dressing toward curry spice and dried fruit. Each deserves its own article rather than being crowded in here.