The Croque-Provençal is the croque-monsieur after it has spent a summer in the south. The structural bones are the same, pain de mie, cheese, the broiler, but the register shifts toward the Mediterranean. Sliced tomato goes into the middle, herbes de Provence get worked into the béchamel or scattered over the top, and black olives, sometimes a smear of tapenade, push the whole thing toward Nice and away from Paris. The cheese often moves with the geography too, from Gruyère toward a milder chèvre or a young Tomme, which sits more comfortably alongside the tomato and herbs.
The tomato is the variable that decides whether the sandwich works. Too watery and it floods the bread; the better versions use tomatoes that have been salted and drained, or oven-dried just enough to concentrate them. The herbs do the rest of the work, thyme and rosemary and savory carrying the scent the standard croque gets from nothing but toasted cheese. It is a lighter sandwich than its northern parent, eaten in warmer weather, and it reads as a café lunch on a terrace rather than a zinc-counter snack.
The Croque-Provençal belongs to the broad family of regional croques that swap the Parisian cheese-and-ham logic for a local one. The Croque-Monsieur tradition that frames all of them, including the béchamel technique and the codified naming of each variant, is covered in its own article. The Provençal version's contribution is to show that the format survives a full change of climate: take the same bread, the same broiler, and the same idea of binding a sandwich with melted cheese, and the south rebuilds it around tomato and herb without losing what makes it a croque.