The Croissant Garni is what happens when a French breakfast pastry decides it would also like to be lunch. A buttery, laminated croissant is split lengthwise like a roll, warmed gently, and stuffed with something savoury — most often jambon-fromage, sometimes saumon fumé with whipped cream cheese, occasionally avocat and mozzarella in the summer-menu version that the cafés around the 11ᵉ have made their own.
The sandwich works because of what the croissant itself is. The laminated layers — between sixty and a hundred of them, depending on the bakery — give you a texture no other French bread will: crisp shatter on the outside, soft butter-cream on the inside, holes large enough to catch melted cheese, structure rigid enough to hold a filling without collapsing. A baguette can take more weight, but it won't dissolve into the filling the way a croissant does. That dissolution is the point.
The Croissant Jambon-Fromage is the canonical version, and the French version really is just ham and cheese. The Italians and the Americans will add tomato, basil, pesto, mustard, and a glaze of balsamic, and the result is a different sandwich, but the French version trusts the croissant to do most of the work and asks the filling to stay out of its way. It is, accordingly, a sandwich that lives or dies at the bakery counter that morning, and not really portable past about 11 a.m.