1. In a Dutch oven, saute onion and garlic in oil until soft. Add well scrubbed mussels, wine, parsley sprigs, thyme, and bay leaf. Sprinkle with ¼ teaspoon pepper.
2. Bring to a boil, cover, lower heat, and simmer for 6 to 7 minutes, shaking pan, until mussels open. Transfer mussels to a large bowl.
3. Strain cooking liquid through cheesecloth, and boil to reduce to 1 cup.
4. In another bowl, combine ¼ teaspoon pepper, vinegar, mustard and salt. Pour in olive oil in a slow stream.
5. Add celery, parsley, and tarragon, and stir the mixture into the reduced cooking liquid.
6. Pour over the mussels and serve at room temperature.
