The Filet-O-Fish is the rare fried-fish sandwich whose bun is steamed instead of toasted, and that single inversion defines it. Almost every fried sandwich is built to protect a crisp crust, which usually means a sturdy or griddled bun and a sauce that insulates rather than soaks. This one goes the other way: a soft, square bun is steamed until it is pillowy and faintly damp, deliberately giving up all structure so it offers zero resistance against a delicate breaded fillet. The sandwich is engineered around softness on every surface, which is the opposite of how a fried fish sandwich is normally assembled.
The craft is in the two precise portioning decisions that hold it together. The fish is a rectangular breaded white fillet, fried so the coating sets but cut to the footprint of the bun rather than overhanging it, so the soft bread and the soft fillet stay in proportion. The cheese is the detail that gives the build its identity: a single half-slice of American cheese, laid on so it covers only part of the fillet, melted by the residual heat into a thin seam rather than a full blanket. A whole slice would overwhelm a mild fish; half a slice supplies just enough fat and salt to bind without burying it. Tartar sauce, a cold, bound, slightly acidic emulsion flecked with relish, supplies the only sharp element in an otherwise gentle sandwich and does the structural job of keeping the fish from reading as flat. Because the bun is steamed and the sauce is wet, the whole thing is assembled to be soft and cohesive rather than crisp, and it is built to a fixed pattern so the half-slice and the sauce land the same way every time.
The variations are a matter of scaling the same soft frame. A double stacks two fillets and a second half-slice while keeping the steamed bun; a spicier reading swaps the tartar for a peppery sauce; the broader American fried-fish sandwich on a crusty roll with a full slice of cheese is the close relative that abandons the steamed-soft logic entirely. Each of those deserves its own article rather than being crowded in here.