The Sandwich Lamproie is a Bordeaux curiosity, and the filling is a dish before it is ever a sandwich. The lamproie is the lamprey, an eel-like river fish, prepared à la bordelaise: cooked slowly in red wine with leeks and the fish's own blood until the sauce thickens dark and the flesh turns rich and dense. Spooned into bread, it is closer to a braise than a fillet, deeply savory, winey, and gamey for a fish. The build is a length of crusted bread, often split and lightly buttered, packed with the wine-braised lamprey and a little of its sauce. The defining element is that à la bordelaise braise, an unusual river fish cooked into something heavy and red, set in bread that has to carry it.
The logic follows from what is being packed in. This is not a clean, dry fillet but a wet, slow-cooked braise, so the sandwich behaves like a stew folded into bread: the sauce wants to soak in, the flesh is tender and falls apart, and the whole thing is rich rather than light. That sets a hard constraint on the bread, which needs a real crust and a firm crumb to hold a wine-dark braise without going to mush, and on the portion, because too much sauce drowns the loaf within minutes. The fish is intense and gamey, so it asks for restraint around it rather than competition; a single acidic note keeps the red-wine richness from coating the palate. It is best slightly warm and eaten soon, never hot and never fully cold, while the bread still has structure and the braise has not loosened it.
Variations stay close to the Bordeaux table rather than leaving it. More or less of the braising sauce shifts it between damp and contained; the leeks and aromatics of the bordelaise come forward or recede; a sharper pickle or a leaf cuts the richness. Each is a recognizable adjustment of the same wine-braised-fish idea. It belongs with the fish builds the catalog groups under Baguette Poisson, and its particular contribution is a rare river fish cooked à la bordelaise into a heavy red braise, a regional curiosity the bread exists to contain rather than lighten.