The Croque-Champignons is the croque-monsieur rebuilt so that mushrooms, not ham, carry the sandwich. It is often the vegetarian option on a café menu that wants one without rethinking its whole kitchen: take the standard pain de mie, béchamel, and Gruyère, drop the jambon de Paris, and replace it with a generous layer of sautéed champignons de Paris. The result keeps the croque's familiar architecture while shifting its center of gravity from cured pork to something earthier and lighter.
The technique is entirely about driving water out of the mushrooms before assembly. Champignons de Paris are about ninety percent water, and a croque cannot survive a wet middle. Cooks sauté them in a hot pan until they have given up their liquid and started to brown, often finishing with a little garlic, a splash of white wine reduced to nothing, or a spoon of crème fraîche to bind them. Underseasoned mushrooms make a flat sandwich, so the better versions push the salt and add a herb, usually parsley or thyme, to give the filling the savory weight the absent ham would have provided.
The Croque-Champignons sits beside the Croque-Forestier, which is essentially the same idea under a more rustic name, and shades into the Croque-Végétarien when more vegetables join the mushrooms. All of them are filling-swaps on a fixed frame. The Croque-Monsieur tradition that frames the whole group, and the béchamel-and-broiler logic that holds it together, has its own article. The mushroom version's place in that tradition is as the most credible vegetarian croque: a sandwich that does not feel like the ham was simply removed, because something with its own savory presence was put back in.