The walnut and celery sandwich is a Waldorf salad reduced to its crunch and folded into a tea finger, and what defines it is texture rather than richness. Chopped walnuts and finely diced celery are bound with cream cheese and spread on soft crustless bread: the cream cheese is the mortar, but the point of the sandwich is the double crunch running through it, the brittle snap of walnut and the wet, fibrous snap of celery against an otherwise soft, mild filling. It is the salad's contrast translated to bread, the Waldorf without the apple and the leaf, kept small and trimmed because this belongs to the afternoon-tea tray where the brief is delicacy, not bulk.
The craft is the cut, the bind, and moisture control. The walnuts are chopped fine enough to distribute through every bite but not so fine they turn to a damp paste that loses the crunch that is the whole reason they are there; the celery is diced small and, ideally, the stringier outer ribs are peeled, because a long fibre of celery in a soft filling drags out instead of snapping. Celery carries water, so it is best dried after cutting and the cream cheese kept slightly firm, since a loose bind plus a weeping vegetable runs into the crumb and the finger goes limp before it reaches the table. The cream cheese is spread to the edges to waterproof the bread and supply the gentle, slightly tangy body the crunch sits in, the soft white is sliced thin, and the crusts are cut away after assembly so nothing with chew resists a filling chosen for its snap. It is made close to serving, because a tea sandwich with a wet vegetable in it does not keep.
The variations are the rest of the Waldorf and the wider walnut canon, each its own pairing. Adding diced apple and a few grapes pulls it back toward a full Waldorf reading; date and walnut shifts it sweet; a blue cheese mashed through in place of plain cream cheese turns the mildness sharp; a little grated celeriac deepens the vegetal note. Each is a distinct tea sandwich rather than a tweak to this one, and each deserves its own article rather than being crowded in here.