The Sandwich Salade Lyonnaise folds a composed bistro salad into bread, and the curiosity of the thing is whether the salad survives the move. The salade lyonnaise is curly frisée dressed in a sharp mustard vinaigrette, scattered with hot rendered lardons, and crowned with a soft-poached egg whose yolk is meant to break and slacken the dressing. The build is a sturdy crusted loaf split lengthwise: the frisée and warm lardons packed into the crumb, the poached egg set in and split so the yolk runs through the leaves, the vinaigrette doing the binding the way it does on the plate.
The logic is that the salad's own architecture has to do the work a sandwich filling usually does. Frisée is the right green for this because it is rigid and bitter: it holds its loft against warm fat and a wet dressing far longer than a soft leaf would, so the sandwich keeps some structure instead of collapsing into a damp wad. The lardons bring salt and rendered fat that the bitter frisée needs to read as balanced, and the broken yolk is the binder, coating the leaves and slicking into the crumb so the whole thing eats as one rather than as loose salad escaping the bread. The constraint is timing and restraint. The vinaigrette must be sharp enough to cut the fat but not so wet that it drowns the loaf, the lardons go in hot so their fat carries, and the sandwich is best within a few minutes of assembly, before the dressing saturates the crust past the point of bite. The bread needs a real crust precisely because the filling is wet and loose and offers no structure of its own.
Variations stay inside the Lyonnais bouchon repertoire rather than wandering off it. A wedge of soft-cooked egg instead of a poached one is tidier and travels better, if less generous when it breaks; more lardons and less frisée reads richer; a few croutons folded in add a dry crunch against the wet dressing. Each holds frisée, lardons, and egg as the fixed trio and adjusts only the proportions or the form of the egg. It sits with the composed-salad builds the catalog groups under Sandwich Végétarien, with the lardons and egg keeping this one firmly on the meat side of that shelf. Its specific contribution is a composed bistro salad held together well enough by its own yolk and vinaigrette to work between bread.