The Croque-Forestier is the croque-monsieur with the forest floor added. Forestier in French cooking signals mushrooms, and in this sandwich a layer of sautéed champignons goes in alongside or in place of the ham, usually with the pan juices reduced so the moisture does not waterlog the bread. The standard béchamel and Gruyère stay, the broiler step stays, and the mushrooms slot in as a third savory element that brings an earthy, slightly meaty note the plain croque does not have.
The execution problem is water. Mushrooms release a great deal of liquid when they cook, and a croque is a sandwich that does not tolerate a wet interior. The better versions sauté the mushrooms hard and dry, often with a shallot and a splash of white wine cooked off completely, before they ever touch the bread. Button mushrooms are the everyday choice; a kitchen with access to chanterelles or ceps will make a markedly better sandwich and usually says so on the menu. A little crème fraîche or a few sprigs of parsley sometimes joins the mushrooms, edging the sandwich toward a duxelles between toast.
The Croque-Forestier overlaps almost entirely with the Croque-Champignons, the difference being mostly one of menu vocabulary rather than recipe. Both belong to the wide field of croques that vary the filling while keeping the béchamel-and-broiler architecture intact. The Croque-Monsieur tradition that frames them, and the way each small swap earns its own name, is covered in its own article. What the forestier version adds is an argument that the croque format is robust enough to carry a vegetable as its centerpiece, provided the cook respects how much water a mushroom is holding.