Croque-Monsieur

Popularly said to have been created around 1910 at the Le Bel Âge café on the Boulevard des Capucines in Paris, though the dish predates this — it appears in print earlier, including an 1891 mention in La Revue Athlétique. The name ("Mister Crunch") was café slang. The dish was a quick savory snack for between-meal hours, designed to use day-old pain de mie and the kitchen's standing béchamel. Proust mentions it in À l'ombre des jeunes filles en fleurs (1918), confirming its café status by then.

First on a Paris café menu in 1910 at Le Bel Âge on the Boulevard des Capucines; Proust mentioned it in 1918's À l'ombre des jeunes filles en fleurs. Béchamel is the trick — without it, it's just grilled cheese.

Two slices of pain de mie pressed around ham and Gruyère, the whole thing capped with béchamel and more grated Gruyère, then broiled until the top is brown and bubbling. The béchamel keeps the inside molten; the bread underneath turns dark and butter-fried on the bottom. Add a fried egg on top and it becomes a Croque-Madame.

Béchamel is the trick. Without it, the sandwich is just grilled cheese with ham; with it, you get the gratin top and an internal sauce that keeps the cheese from going stringy. Use Gruyère, not Emmental — Gruyère browns; Emmental melts but stays pale. Butter both outer faces of the bread before pan-toasting, then finish under the broiler so the top crusts before the inside dries.