The Croque-Bleu pushes the Croque-Monsieur past the point where the cheese is supposed to be a quiet partner. Roquefort or Bleu d'Auvergne replaces the Gruyère, and the sandwich's whole flavour balance reorients around it. The béchamel, which in the canonical version smooths and binds, here acts more like a buffer: it dilutes the blue cheese's salt and acetic-acid bite enough that the bread can still register. The ham is often dropped entirely, or thinned to a single slice, on the principle that a strong blue is doing more than enough work without a competing protein.
The cheese choice matters more in this form than in any of the other regional croques. Roquefort, the AOP sheep's-milk blue from the limestone caves of the Aveyron, is the most assertive option: salty, crumbly, ammonia-edged at the rind, with a flavour that asserts itself the moment it hits heat. Bleu d'Auvergne, a softer cow's-milk blue from the Massif Central, melts more cooperatively and reads slightly milder, which makes it the more common choice in restaurant settings. Fourme d'Ambert sits between the two, with a tighter paste and a more buttery profile. Whichever blue is chosen, the sandwich rewards a thin layer of crushed walnut or a thin slice of pear underneath the cheese, both of which the canonical croque would never tolerate but which the blue version genuinely needs to keep its sweetness in balance. A drizzle of honey on top after broiling pushes the form toward dessert in a way some cooks embrace and others avoid.
The version sits among the cheese-forward croques that include the Croque-Chèvre, the Croque-Savoyard with reblochon, and the various single-cheese constructions that lean on a regional cheese rather than the Gruyère-and-ham default. The broader Croque-Monsieur family absorbs the Croque-Bleu as a legitimate variant despite its considerable distance from the canonical form, partly because the blue cheese tradition is deeply embedded in French food culture and partly because the croque architecture genuinely accommodates it. The sandwich is most often found at brasseries in the south-central regions where the blue cheeses originate, and only occasionally in Paris, where the assertion of flavour can feel out of place against the more restrained Parisian palate.