The Croque Végétarien is the Croque-Monsieur with the jambon de Paris taken out and a vegetable that can survive the broiler put in. It is a more recent addition to the brasserie chalkboard than its cousins, and the better versions take the absence of the ham seriously: this is not a Croque-Monsieur missing its protein, but a sandwich rebuilt around a different center. The classic substitution is a layer of grilled aubergine and courgette, salted in advance so they release their water before the bread sees them. A swipe of tapenade or a thin layer of pesto under the cheese carries the salt and the umami that the ham would have brought. Gruyère or Comté stays on top, because the broiler logic is non-negotiable: a Croque without the lacquered cheese crown is something else.
The cooking technique is what keeps the sandwich from collapsing into a wet open-face. Vegetables go on first, the béchamel goes over them, the cheese caps the structure, and the broiler does its work fast enough that the bread crisps before the moisture migrates. Cooks who skip the pre-salting end up with a sandwich whose bottom slice has given up by the second bite. The best Croque Végétariens are eaten in the same brasseries that built the original, the kind of place where a busy lunch service still finds time to grill the courgette to order. A small bitter salad on the side, dressed in mustardy vinaigrette, gives the plate the acid it needs to push back against the cream.
The variations track regional vegetable supplies. Provençal versions lean on grilled poivrons and a layer of confit garlic. Alsatian kitchens have been known to stack thin-sliced champignons over a smear of crème fraîche and shallot. Spinach-and-goat-cheese versions read more like a tartine that has decided to close itself. The cross-link goes back to Croque-Monsieur, where the full family of variants lives. The vegetarian branch is the one that has the most room to expand, because the classic Croque had only ever made one decision about its protein, and once that decision is loosened the form is wide open.