The Sandwich Ficelle Picarde is a regional baked dish carried into bread, and the name is a false friend: there is no thin loaf here. The ficelle picarde of Picardy is a savory crêpe, rolled around a filling of ham and mushrooms bound in a creamy sauce, then laid in a dish, blanketed with more cream and grated cheese, and baked until the top browns and the whole thing sets. As a sandwich it works the way the other plate-in-bread builds work: the ficelle picarde is cooked, sliced or kept as a roll, and tucked still warm into a length of crusted bread that acts as the plate and the handle at once.
The craft is in heat and structure. The filled crêpe is soft, rich, and gratinéed, closer to a cannelloni than to a cold cut, so the sandwich lives or dies on temperature: warm enough that the cream stays loose and the cheese pulls, never so hot that the sauce slumps out the open end and never fridge-cold, where the béchamel-style binding turns to paste. The bread does real work. It has to have a firm crust and enough crumb to soak the cream the crêpe gives off without collapsing, because the filling brings no structure of its own. The mushrooms supply an earthy savor, the ham a cured salt, the cream a fat that the wheat absorbs and steadies. Cut the portion wrong and the bread surrenders by the fifth bite; keep it modest and the loaf holds.
This is a Picardy specialty before it is a sandwich, the kind of warm gratin a cook makes ahead and a sandwich extends into the next day's lunch.
Variations stay inside the Picard and northern repertoire of cream-and-cheese baked things rather than wandering off it. The same treatment takes a flamiche, the leek tart of the same region, or other gratinéed fillings whose richness the bread can carry. Each is a baked dish given a crust to hold it. The Sandwich Ficelle Picarde belongs with the regional dishes folded into bread that the catalog groups under Plat-en-Sandwich. Its specific contribution there is the gratin: a crêpe baked under cream and cheese, loaded warm into bread that has to behave like a plate.