Paillards are quickly cooked, thin scallops of meat. In this recipe tender pieces of turkey breast are marinated and used in a simple, savory sandwich.
Remove tenderloin from turkey; cut tender in half lengthwise. Cut breast into 3/4-inch-thick medallions. Cut open two sides of a resealable plastic bag. Place 1 medallion in bag; pound to a 1/4-inch thickness with a meat pounder. Transfer to a nonmetal container. Repeat with remaining turkey. Add marinade. Cover with plastic wrap, and refrigerate for 2 hours.
Preheat a grill or grill pan to medium hot. Brush onion, tomatoes, and bread with oil. Season onion and tomatoes with salt and pepper. Grill onion until browned and soft, about 2 minutes per side. Set aside on a plate. Grill tomatoes without turning until lightly browned, about 2 minutes. Grill bread until just toasted, 1 to 2 minutes.
Remove turkey from marinade; season with salt and pepper. Grill until browned and cooked through, 1 to 2 minutes per side. Layer bread with onions, turkey, Parmesan, basil, tomatoes, and watercress; top with remaining bread. Serve.