Cream cheese and smoked salmon is the rich member of the tea-tray cream-cheese family, and it is defined by one partner set against a spread doing structural work. Cream cheese is taken to the edges of bread and smoked salmon is laid over it, often with capers or dill, and that is the sandwich. The cheese is the body and the mortar both; the salmon is the single defining partner, a cured, oily, salt-and-smoke richness that the soft, mild cheese is there to carry and round off. The pairing is the whole idea. The salmon alone is intense and slippery and has nothing to bind it; the cheese alone says little. Together the cheese tempers the salmon and the salmon gives the cheese a reason to be there.
The craft is balancing two rich things so they read as one and not as a slick. The cream cheese does structural duty here in the way butter does elsewhere: spread firm and even to the edges, it binds the slices, seals the crumb against the oil of the fish, and gives the salmon a stable bed so it does not slide out under pressure, which makes an even, edge-to-edge spread the core mechanics of the build. The salmon is sliced thin, to translucence, so it folds and layers rather than sitting as a heavy slab that overwhelms the cheese and unbalances the bite. A few capers or a little dill are worked in as a sharp, measured counter, set against the richness rather than scattered for show, because two soft rich layers with no acid read as cloying. The bread is soft and plain, white or brown, because the filling carries all the flavour and a loud crust would only argue with it. Cut crustless and small for the tea tray, it stands on the salmon being sliced thin and the cheese being spread to the edge.
The variations are the rest of the cream-cheese tea shelf, each defined by the partner set against the same structural spread. Cream cheese and cucumber sets a cool, water-crisp slice against it instead; cream cheese and chive works a green onion herb through; cream cheese with walnut or celery adds a dry crunch. The plain smoked salmon sandwich drops the cheese for brown bread and butter. Each deserves its own article rather than being crowded in here.