The Croissant au Jambon is the French breakfast pastry conscripted into sandwich duty. A baked croissant is split lengthwise and filled with a slice or two of jambon de Paris, sometimes with a little butter, sometimes with a thin layer of cheese, and either eaten cold or warmed through so the butter in the laminated dough softens again. It occupies the café counter at the hour when breakfast and lunch overlap, the thing you order at eleven when a pain au chocolat feels too sweet and a full jambon-beurre feels too committed.
The croissant is an unusual sandwich bread because its structure is the opposite of a baguette's. A baguette has a sturdy crust and an open crumb built to hold a filling without falling apart. A croissant is laminated, fragile, and rich with butter, which means it shatters into flakes the moment it is bitten and cannot carry a heavy or wet filling without disintegrating. This is why the classic version stays minimal: ham and maybe butter, nothing that would overwhelm the pastry or turn its layers to paste. The croissant's own richness is doing flavor work that a plain bread would leave to the filling.
The most common upgrade is the Croissant Jambon-Fromage, which adds Gruyère or Emmental and is usually warmed so the cheese melts into the layers, at which point it sits very close to a croque-monsieur built on a croissant instead of pain de mie. Sweet and almond-filled croissants are a different tradition and not sandwiches in any useful sense. The broader Croissant Garni family covers the full range of filled-croissant formats and where the line sits between a viennoiserie and a sandwich. The plain ham version is the baseline of that family: the least done to the pastry, and the clearest demonstration of why a croissant resists being treated like bread.