The fish sandwich tradition in France runs along the coasts and inland up the rivers, and the version on the baguette varies by which water the fish came from. On the Atlantic and Breton coasts it's smoked or grilled mackerel, sardine, herring, anguille fumée. On the Mediterranean it's tinned tuna folded with capers or anchovy paste with butter on toasted bread. In Paris the smoked salmon sandwich — saumon fumé, beurre, ciboulette, citron — is the version every brasserie carries between 7 and 10 a.m.
The oily fish — sardine, hareng saur, maquereau fumé — pair with bread and butter the way they pair with a glass of crisp Muscadet, which is to say without effort. A sardine sandwich done well is a sardine pulled from its tin, the oil shaken back onto the buttered bread, a few drops of lemon, and a turn of black pepper. The technique is barely a technique. The work was done by whoever caught and tinned the fish.
The tuna sandwich gets a separate treatment. The Niçois prefer tinned tuna in olive oil layered with anchovy and tomato (see the Pan Bagnat); the Parisian sandwich-shop version folds tuna with mayonnaise, capers, and lemon zest into a baguette and is closer to a salad-on-bread. Both work; both are eaten at the same lunch counters; the only sandwich that doesn't work is the one made with tuna that was tinned in water and shows up at the counter pre-mixed with too much mayo and too little salt.