- 1/2 stick unsalted butter
- 1 medium onion, chopped
- 2 ribs celery cut into 1/4 cup dice
- 1 (14- to 15-ounce) can diced tomatoes
- 3 garlic cloves, finely chopped
- 1 teaspoon chopped fresh thyme
- 1 bay leaf
- 1/2 teaspoon salt
- 1/4 teaspoon black pepper
- 2 cups (16 ounces) lager such as Harp (pour beer slowly into measuring cup; do not measure foam)
- 2 pounds mussels, scrubbed well and beards removed
- 1 tablespoon Dijon mustard
- 2 tablespoons heavy cream
- 1/4 cup flat-leafed parsley, chopped
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 to a boil. Add mussels and cook, covered, stirring occasionally, until mussels open wide, 4-6 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. Makes 4 first course or 2 main course servings.