Coarsely grate 1 pound peeled celery root. Toss with 1 tablespoon salt and 2 tablespoons warm water in a colander; let sit 20 minutes. Rinse and drain. Whisk the juice of 1 lemon and 1/4 cup each dijon mustard and sour cream in a bowl, then whisk in 1/4 cup olive oil. Add the celery root, 1 teaspoon salt, and pepper to taste. Chill. Add 1/4 cup chopped parsley before serving.