Complete Gratin

Gratin complet de pommes de terre (complete potato gratin) teaches timing by proportion. The one true thing to know before touching a pan is this: the potatoes, the sauce, and the crust must finish together. Too much sauce leaves cooked potatoes swimming beneath a finished crust; too little disappears while the potatoes are still firm. Match the quantity of sauce to the thickness of the food and the heat of the oven. That is the dish.

The original method assumed a saucier balancing reductions, a stockpot that never cooled, and a salamander ready to correct the surface at the final moment. At home, a shallow gratin dish, evenly sliced potatoes, measured sauce, and the upper third of the oven do the same work. A brief pass beneath the broiler replaces the salamander only if the crust needs it. The separate pan reduction used to repair careless scaling is brigade scaffolding and goes; the simultaneous finish stays. One cook, one stove, one evening.

When it is right, a knife slips through the potatoes without resistance, the duxelle-rich sauce bubbles thickly at the rim, and the chapelure forms a crimped, butter-gold crust rather than a loose scattering of crumbs. The one step that decides it is calibration: slice every potato 3 mm thick and begin with sauce that coats a spoon while still pouring freely.