Nothing here is bound by its own rendered fat, and that single difference separates this rillettes from every pork or duck version. Rillettes de maquereau are mackerel, the oily Atlantic fish, flaked and worked into a soft spread held together not by a slow-cooked meat fat but by a loosening agent folded through it: crème fraîche, a little butter, lemon, often a few capers or chopped shallot. The result is lighter, paler, and looser than a potted-meat rillettes, fishy in the assertive way only an oily fish is, but lifted and cut rather than dense and warming. The build is a length of baguette or a split crusted loaf, a thick layer of the mackerel spread along the crumb, and little else, so the fish stays in front.
The logic follows from the fish oil and the dairy. Mackerel is rich, but its richness coats the palate and goes dull or sour if nothing sharp is set against it, which is exactly the job the crème fraîche, the lemon, and the capers do from inside the spread rather than alongside it. The acid is built in, not a side condiment, because without it the oil flattens by the last bite. The spread is soft and yielding and brings no structure, so the bread still has to have a real crust to hold its shape, but unlike a fat-bound meat rillettes it does not slick the crumb into one mass; it sits lighter and wetter. It eats cold and stays bright that way, where a pork rillettes would need to warm to soften, this one wants the chill to keep the dairy and the lemon crisp.
Variations move along the smoke and the loosening agent. Hot-smoked mackerel folded in reads denser and saltier; fresh-cooked fish keeps it cleaner and milder; more crème fraîche softens the oil while more lemon and caper sharpen it. Each holds the loosened, flaked fish as the fixed point and adjusts richness against acid. The Sandwich Rillettes de Maquereau belongs with the cured-meat and spread sandwiches the catalog groups under Sandwich Saucisson & Charcuterie, the tradition that runs across France's regional curing shelves. Its specific contribution is a rillettes bound by crème fraîche and lemon rather than its own fat, lighter and sharper than any potted meat.