Hot pepper vinaigrette is the ideal accent for this southern Italian seafood salad. When fresh crabmeat or shrimp are available, we like to to use them. Smoked trout is a rich and interesting alternative; just be sure to remove all of the bones.
Toss the onions with ¼cup of the vinaigrette and let marinate to soften them a bit.
Toss the watercress with enough vinaigrette to lightly coat and arrange in a bed on each of 2 salad plates. Top with the marinated onions. Toss the shellfish with most of the remaining vinaigrette and add to th