The Tartine Parisienne is the café version of the open-face slice: a length of split baguette, butter, and ham, eaten cold at a zinc counter as a light lunch. The defining element is the format. Where the sweet tartines are a single short slice eaten by hand at home, this one runs the length of a baguette cut lengthwise and is eaten with a knife and fork at a table, which is what makes it a café order rather than a kitchen snack. It is the savory midday tartine, jambon de Paris and a demi-sel butter laid open on bread, and the moment is the middle of a working day, not the morning or the afternoon goûter. The region recorded for it is Paris.
The craft is what an open length of baguette can carry that a closed sandwich handles differently. With no top half pressing down, the butter and ham sit exposed, so the proportions are read by eye and have to be right on sight: butter spread the full length to the crust, ham laid flat in even folds so no stretch of bare bread shows. The baguette is split along its length and left with its full crust, which is the structural point, since a single open half has to stay rigid under the topping and a baguette crust does that where a soft loaf would sag. The butter does the same bridging work it does on a breakfast tartine, carrying the salt of the ham into the wheat, but here the slice is long enough to eat in measured cuts rather than in a few bites. It is best soon after it is built, while the crust is still firm and the butter has not gone soft under the ham.
The variations are the surrounding family, each a single change for its own moment. Swap the ham for cheese left cold and you have the Tartine au Fromage; run that cheese under heat instead and it is the Tartine Gratinée. Strip the savory topping back to butter alone and it is the plain Tartine Beurrée the whole family rests on. The ham and the addition of a cornichon or a leaf vary by the counter serving it. The Tartine Parisienne sits in the open-face tradition the catalog groups under Tartine, and its particular contribution is the baguette-length café format, an open sandwich eaten with a fork at the zinc.