To cook the seafood, combine 8 cups of water with the white wine vinegar and salt in a large saucepan and bring to a boil. Add the shrimp and cook for 2 minutes only. Remove with a slotted spoon. Bring the water back to a boil and cook the scallops for 4 to 5 minutes, until cooked through. Drain.
Bring 1/2 cup 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.) Drain. Remove the mussels from the shells and discard the shells. Drain all the cooked seafood and place it in a large bowl.
To make the sauce, 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. 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.
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