A lean tinned base sets the plain, pantry-built cousin of the rillettes family apart. Rillettes de thon are tinned tuna, drained and flaked fine, then worked into a spread held together by crème fraîche, soft butter, or a little mayonnaise, since the fish brings almost no fat of its own to bind it. There is no slow cook and no rendered fat: the tuna is already cooked in the tin, and the spread is an assembly rather than a confit, sharpened with lemon, sometimes mustard, chopped shallot, or capers. The build is a length of baguette or a split crusted loaf, a thick layer of the tuna spread along the crumb, and little else, so the fish stays in front.
The logic follows from how lean the base is. Tinned tuna is dry and mild compared with an oily mackerel or a fatty pork, so the binder is not optional: the crème fraîche or butter supplies the body the fish lacks, and the lemon and mustard supply the lift the bland flake needs. Get the binder right and the spread is smooth and savory; too little and it reads dry and pasty, too much and the tuna disappears into the dairy. The spread brings no structure of its own, so the bread still has to have a real crust to hold its shape, but it sits light in the crumb rather than slicking it the way a fat-bound meat rillettes does. It eats cold and keeps that way, the chill holding the binder firm rather than slack.
Variations move along the binder and the sharpening. More crème fraîche makes it rounder and milder; mustard and capers drive it sharper and brighter; a little chopped egg or shallot gives it texture against the fine flake. Each holds the lean tinned tuna as the fixed point and adjusts only its body or its bite. The Sandwich Rillettes de Thon 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 the budget rillettes, a lean tinned fish that brings no binding fat and leans on dairy and acid to become a spread at all.