The Tartine is the sandwich that doesn't quite obey the rules of the form — there's no second slice — but it has held its place in the French daily diet long enough that it gets its own seat at the table. A single slice of pain de campagne (or pain au levain, or rustic seigle) is grilled, the toppings are layered with intent, and the result is eaten with a knife and fork or, in the truest spirit, with hands on a café terrace.
The savoury tradition runs from the breakfast tartine of butter and a smear of jam to the lunch tartine that carries a full salad's worth of ingredients: smoked salmon with whipped cream cheese, pickled red onion, capers, dill; warm goat cheese on toasted seeded bread with honey and walnuts; mushroom duxelles under a poached egg. The sweet tradition belongs mostly to children and to the older French who remember Nutella before it became a global brand, but the savoury tartine is where the form earns its keep.
At its best it's an exercise in restraint — three ingredients on a slice of well-baked bread, none of them trying to be the loudest. At its worst it becomes a vehicle for whatever was in the fridge. The line between the two is whether the bread can carry what's on it without giving up its texture by the third bite.