In a large pot, bring 2 inches of water to a boil. Add the clams, cover and steam over high heat until they open, about 5 minutes. Transfer the clams to a bowl.

In a large enameled cast-iron casserole, melt 2 tablespoons of the butter. Add the leeks, season with salt and cook over moderate heat, stirring occasionally, until softened, 8 minutes.