The Sandwich Bistrot is less a fixed recipe than a register: the sandwich as the bistro counter makes it, plated next to a drink rather than wrapped to go. There is no single defining filling, because the defining thing is the context. A bistro builds it from whatever the kitchen already keeps for its plates: a length of baguette or a few slices of pain de campagne, good butter, and then ham, a country pâté, a hard cheese, a slice of cold roast, dressed with cornichons and a stripe of mustard. What makes it a Bistrot sandwich rather than a counter sandwich is that it is assembled to be eaten sitting down, slowly, with a glass of wine or a coffee beside it, and the components are chosen to hold up to that pace.
The logic follows from the table. A sandwich eaten standing at a kiosk has to be portable and fast; a sandwich eaten at a bistro table with a drink can be looser, richer, and cut with a knife if it wants to be. That changes the build. The bread can be a softer pain de campagne rather than a rigid baguette because it does not need to survive a coat pocket; the filling can run to a thick slab of pâté or a soft cheese because it will be eaten in minutes, not carried for an hour; the acid element, the cornichon or the mustard, matters more because it is doing the same job a glass of cool wine does, cutting the fat between bites. Served fresh on bread from that morning the whole thing is unhurried and generous; held in a case it loses the point, because the point was that it was made for the table it is sitting on.
Variations are really just the bistro's own larder showing through. A board with rosette and butter is one reading; pâté de campagne with cornichons is another; a hard mountain cheese with a few walnuts is a third; cold roast beef with mustard is a fourth. None of these is the canonical Bistrot sandwich, because the category is the setting rather than the filling, and each kitchen fills it from its own shelf. The Sandwich Bistrot belongs with the context-named sandwiches the catalog groups under Regional Specialty Sandwiches. Its specific contribution is not an ingredient but a frame: the sandwich built to be eaten at a table with a drink, where richness is allowed and speed is not the constraint.