Ingredients
Method
Heat butter in a wide 5- to 6-quart heavy pot over moderately high heat until foam subsides, then cook onion, celery, tomatoes, garlic, thyme, bay leaf, salt, and pepper, stirring occasionally, until vegetables are softened, about 4 minutes.
Add beer and bring just to a boil. Add mussels and cook, covered, stirring occasionally, until mussels open wide, 4 to 6 minutes, transferring them to a bowl as they open. (Discard any mussels that remain unopened after 6 minutes.) Remove pot from heat. Stir together mustard and cream in a small bowl, then add mixture along with parsley to hot broth and whisk until combined. Discard bay leaf. Serve sauce over mussels.
