Everything that defines the French lunch counter's everyday fish sandwich starts with a can opener. Canned tuna is flaked and folded with mayonnaise, brightened with lemon, often loosened with chopped cornichon, caper, or onion, and packed into a split baguette. There is no cooking and almost no technique; the entire sandwich is decided by the quality of the tin and the proportions of the bind.
The logic of the sandwich is balance against blandness. Tuna out of a can is mild and, if it is the kind packed in water and over-drained, on the dry side, so the mayonnaise is doing structural work: it binds the flakes into a cohesive layer and restores the fat that gives the filling body. Too little and the sandwich is dry and dull; too much and it slumps into paste and the bread goes wet. Lemon and a little salt pull the whole thing back into focus, because tuna mayonnaise without acid reads flat fast. The sharp additions, cornichon, caper, onion, are not garnish but counterweight, cutting the richness and giving the soft filling some bite. Tuna packed in olive oil rather than water makes a rounder, more flavorful base and needs less mayonnaise to carry it, which is the single change that most improves the sandwich.
The bread does the structural work the soft filling cannot. A baguette with a firm crust and an open crumb holds the dressed tuna without going slack, and the filling is mounded loose rather than packed dense so it does not compress into a brick. There is no warm component, and the sandwich is best made close to service: tuna mayonnaise sitting in bread soaks the crumb and dulls within the hour.
Variations move along the tin and the additions. Oil-packed tuna for a richer base, a crème fraîche bind for something lighter and tangier, a Niçois-leaning version with egg, olive, and anchovy that pushes it toward a salad-on-bread. It belongs with the fish sandwiches the catalog groups under Baguette Poisson, and its specific contribution is ubiquity: the default tinned-fish sandwich whose quality is set almost entirely by the tin and the ratio of bind to fish.