2) Bring 100ml of water to a boil in the same saucepan and toss in the mussels. Return to a boil, cover, and steam for 3 to 5 minutes, until they're all opened. Discard any that remain unopened after 5 minutes.
3) Drain the mussels, remove them from their shells and discard the shells. Place all the cooked seafood in a large bowl.
For the salad:
1) To make the vinaigrette, heat the olive oil in a medium saute pan and add the thyme, garlic and lemon zest. Cook over low heat for 1 minute. Off the heat, add the lemon juice, mustard, vinegar, salt and pepper.
2) Pour the hot vinaigrette over the seafood. Add the celery and parsley and toss well. This salad can be served immediately, but it is best when allowed to sit, refrigerated, for 1 to 2 hours. Sprinkle with salt and toss with sliced lemon.