The Sandwich Œuf-Mimosa is the most worked of the French egg sandwiches, and the technique is what names it. Œuf mimosa separates the cooked egg: the white is filled or bound with a mayonnaise-rich mixture, and the yolk is pressed through a sieve into fine bright crumbs that are scattered back over the top, a yellow dusting that the name borrows from the mimosa flower. Carried into bread, that means a filling of dressed white with sieved yolk worked through it and a final fall of crumbed yolk across the surface. The build is a length of bread, that two-part egg preparation spread or layered inside, salt and pepper, sometimes a little herb. The sieved yolk is the signature, and it is what separates this from both the sliced and the plain salad versions.
The logic is the logic of one ingredient delivered in two textures at once. The bound white is soft and rich; the sieved yolk is dry and powder-fine, almost like a savoury dusting, and the contrast between the two is the whole sensory event rather than a garnish. Because the yolk is loose crumb rather than paste, it sits on top and through the filling without weighing it down, which keeps the sandwich from reading as a single uniform mass the way a plain egg salad can. The mayonnaise in the white supplies the fat, so no separate carrier is needed underneath, but seasoning is structural, since under-salted egg of any treatment tastes of little. The bread wants a soft crumb and a restrained crust, because a tender filling layered in two textures is easily flattened by a loaf that fights it, and the fine yolk crumb is lost if the structure collapses on the first bite.
Variations turn on how the egg is handled. Leave it in plain rounds and you are back at the Œuf Dur register; fold yolk and white together into one cohesive salad and you have an Œuf-Mayonnaise; each is the same egg treated differently, and each gets its own entry rather than being crowded in here. Within the mimosa version the turns stay small: a fold of chive or tarragon into the white, a thread of mustard, a leaf of cress for snap against the soft filling. The Sandwich Œuf-Mimosa belongs with the plant-forward and meatless builds the catalog groups under Sandwich Végétarien. Its specific contribution is the split egg, soft dressed white under a fall of sieved yolk, so the sandwich delivers two textures of one ingredient and the cook's job is to keep that contrast intact.