The bagel with lox is held together by cream cheese, which is doing structural work as much as flavor work. Lox is salt-cured salmon, sliced to translucence, slippery and entirely without grip; a bagel is firm and dense; on their own they would slide apart on the first bite. The cream cheese is spread thick across the cut faces precisely to glue the fish to the bread and to give the capers, red onion, and tomato something to set into. It is the mortar of the whole structure, and the reason the build reads as one thing rather than a fish balanced on a roll.
The craft is in the cut and the restraint of the toppings. A bagel is boiled before it is baked, which sets a tight, chewy crumb and a glossy crust that can carry a heavy, wet load without tearing or going limp, and for this build it is properly left untoasted, because toasting turns the crumb brittle and the point is the bagel's pull against the give of the salmon. The cream cheese is the bind; the lox supplies the salt and the silk; and then a deliberately sharp set of accents earns its place against that richness. Capers add brine and small bursts of acid, thin-sliced red onion adds a pungent bite and crunch, and tomato adds cool and a clean acidity. Each is there to cut the fat of the cheese and the salt of the cure, which is why the build feels balanced rather than heavy despite how rich its two main components are.
The variations are a graded scale of cured fish rather than different sandwiches. Nova is cold-smoked and milder than the assertive salt cure of true lox; sable and smoked sturgeon are richer and smoked; whitefish salad is bound and spreadable. The carrier moves too: a bialy stands in as a softer, onion-centered roll with no hole. Each of those sits on the same appetizing counter and deserves its own article rather than being crowded in here.