The Sandwich Œuf-Mayonnaise puts the bistro counter classic between two slices of bread, and the binding is the whole point. Œuf mayonnaise is hard-boiled egg chopped and folded with mayonnaise into a cohesive salad: yolk and white worked together, the mayonnaise carrying both into a single soft mass rather than leaving the egg in discrete slices. The build is a length of bread, the egg salad spread thick and even, salt and pepper through it, often a little mustard sharpening the mayonnaise, sometimes chopped herb or a leaf of lettuce for snap. The egg is no longer a stack of parts; it is a spread, and that is what separates this from the sliced version.
The logic is the logic of a bound paste behaving as both filling and its own sauce. Because the mayonnaise emulsifies the dry yolk, the salad needs no separate fat carrier under it: the binding already coats the crumb and bridges the egg to the bread in one move. That same richness sets the constraint, since egg salad is soft and uniform all the way through, so the sandwich wants a textural counterweight or it goes one-note by the third bite. Mustard does some of that work by cutting the fat with a sharp edge; a crisp leaf or a few capers do the rest by adding the snap the paste lacks. Seasoning is structural, because under-salted egg salad reads bland and the mayonnaise will not rescue it. The bread wants a soft crumb and a light crust, firm enough to contain a spread that wants to migrate but tender enough not to fight it.
Variations stay close to how the egg is treated. Leave the egg in plain rounds and you have stepped back to the Œuf Dur register; sieve the cooked yolk and mound it over the white instead of folding it through and you are making an Œuf-Mimosa; each is the same egg handled differently, and each earns its own entry rather than being unpacked here. Within the egg-salad version the turns are small: more or less mustard, a fold of chive or tarragon, a layer of cress, a thread of curry for warmth. The Sandwich Œuf-Mayonnaise belongs with the plant-forward and meatless builds the catalog groups under Sandwich Végétarien. Its specific contribution is the egg bound into a spread that carries its own fat, so the sandwich's job is to season it well and give it one sharp or crisp note to push against.