Ingredients

• 1/2 stick (1/4 cup) unsalted butter
• 1 medium onion, chopped (1 cup)
• 2 celery ribs, cut into 1/4-inch dice (1 cup)
• 1 cup drained canned diced tomatoes (from a 14- to 15-oz can)
• 3 garlic cloves, finely chopped
• 1 teaspoon chopped fresh thyme
• 1 Turkish or 1/2 California bay leaf
• 1/2 teaspoon salt
• 1/4 teaspoon black pepper
• 2 cups lager such as Harp (16 oz; pour beer slowly into measuring cup; do not measure foam)
• 2 lb mussels (preferably cultivated), scrubbed well and beards removed
• 1 tablespoon Dijon mustard
• 2 tablespoons heavy cream
• 1/4 cup chopped fresh flat-leaf parsley
• Accompaniment: crusty bread

Instructions

1. 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.
2. 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.