Poach the lobster, cool it, and the cooking is essentially over before the bread enters the picture. That cooled meat is the entire argument: cool, sweet, faintly briny flesh from the claw and tail, laid into split bread with very little else. It is a coastal luxury sandwich, tied to Brittany, the region it is associated with, where the lobster comes off the same water it is eaten beside. The components are spare by necessity: bread, a soft fat or a restrained dressing, the lobster, and almost nothing that would argue with it.
The craft is restraint, because the protein is expensive and delicate and easily buried. The lobster has to stay in recognizable pieces, claw and tail kept whole or in large segments rather than shredded into a salad, so the sweetness and the firm-tender bite read as lobster and not as a filler bound in dressing. The fat is there to carry, not to flavor: a thin film of butter or a light, barely-acidic dressing that lifts the sweetness without covering it. The bread matters more than it looks, because the filling brings no structure and little salt of its own; it needs a crumb soft enough not to fight the tender meat but with enough crust to hold the sandwich together in the hand. This is cold food, assembled and eaten close together, best within a short window of being built, before the lobster loses its chill and its snap.
Variations are mostly questions of dressing and bread, which the catalog treats with the rest of the French fish-on-bread tradition rather than unpacking here: a smoked or oily fish wants a sturdier loaf and an acid, where lobster wants softness and silence around it. Those carry their own treatment rather than crowding in here. The Sandwich au Homard sits at the luxury end of the shelf the catalog groups under Baguette Poisson, and its particular contribution there is the discipline it demands: a sandwich whose whole skill is leaving a costly, fragile ingredient alone.