Turkey Paillard Sandwich

Name: Turkey Paillard Sandwich
Ingredients: 1/2 boneless turkey breast (1 3/4 pounds); Basic Marinade, recipe follows; 1 sweet onion cut in 1/4-inch-thick rounds; 5 plum tomatoes cut in 1/4-inch¿thick rounds; 2 (25-inch) baguettes, halved lengthwise; 1/4 cup extra-virgin olive oil; Coarse salt and freshly ground pepper; 2 ounces Parmesan, sliced paper-thin; 1 cup fresh basil leaves; 1 bunch watercress, tough stems discarded; 1/3 cup extra-virgin olive oil; 2 springs fresh rosemary, crushed; 2 sprigs fresh thyme; 3 large cloves garlic, peeled and smashed; 1 lemon, juiced; 1/2 teaspoon coarse salt; Pinch freshly ground pepper