Clam-and-Oyster Pan Roast

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. Add the mustard greens, thyme, garlic and crushed red pepper and cook, stirring, until the greens are wilted, about 2 minutes. Add the vermouth and clams and bring to a boil. Cover and cook over moderate heat until the clams open, 5 to 10 minutes; transfer the clams to a bowl as they open, discarding any that do not. Add the oysters and their liquor, the cream, parsley, lemon juice, hot sauce and remaining 2 tablespoons of butter and cook just until the oysters start to curl around the edges, 1 to 2 minutes. Discard the thyme sprigs. Stir in the clams and any juices; serve immediately with crusty bread.