A day ahead, prepare the vinaigrette and cook the shrimp; refrigerate separately.
4 teaspoons sherry vinegar
1/2 teaspoon Dijon mustard
1 tablespoon extravirgin olive oil
1 1/8 teaspoons salt, divided
1/4 teaspoon sugar
1/8 teaspoon freshly ground black pepper
6 cups water
1/2 pound medium shrimp, peeled and deveined
2 tablespoons finely chopped fresh parsley
2 teaspoons finely chopped Vidalia or other sweet onion
10 small roasted piquillo peppers
2 cups sliced Belgian endive (about 2 small heads)
2 cups trimmed watercress
1/4 cup vertically sliced onion
How to Make It
Combine vinegar and mustard in a small bowl, stirring with a whisk. Add oil, 1/8 teaspoon salt, sugar, and black pepper, stirring with a whisk until blended.
Combine water and remaining 1 teaspoon salt in a large Dutch oven; bring to a boil. Add shrimp; cook 3 minutes or until done. Drain and rinse with cold water; finely chop. Combine shrimp, parsley, and chopped onion in a medium bowl; stir in 4 teaspoons vinegar mixture. Gently fill each piquillo pepper with about 1 1/2 tablespoons shrimp mixture.
Place endive, watercress, and sliced onion on a large platter, tossing to combine. Top with stuffed peppers, and drizzle with remaining vinegar mixture.