Remove the mussels from their shells and add them to the vegetables. Strain the mussel broth through several layers of cheesecloth. Rinse out the casserole. Return the broth to the casserole.

Add the chorizo and the remaining 1/2 cup of stock and bring to a boil. Add the clams, cover and cook over high heat until nearly all have opened, about 10 minutes. Discard any unopened clams.