Fish Baked with Leeks

This is a dish that is almost too simple to believe, one that combines wonderful textures and flavors with a minimum of ingredients, no added fat, and almost no preparation or cooking time. Like the best minimalist dishes, everything counts here: the fish, the leeks—which remain crisp and assertive thanks to the quick cooking time—and even the wine or stock. The Dijon mustard provides a bit of a kick. You need a tightly covered container to preserve all the liquid and flavors inherent in this dish, but that can be as simple as a pot with a good-fitting lid or a heatproof glass casserole—anything that prevents moisture from escaping.