1. Remove tenderloin from turkey; cut tender in half lengthwise. Cut breast into 1/2-inch-thick medallions. Cut open two sides of a resealable plastic bag. Place 1 medallion in bag; pound to 1/4-inch thickness with a meat pounder. Transfer to a nonmetal container. Repeat with remaining turkey. Add marinade. Chill for 2 hours.

2. Heat a grill or grill pan. 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.

3. Remove turkey from marinade; season with salt and pepper. Grill until browned and cooked through,1 to 2 minutes per side. Layer bread with onion, turkey, Parmesan, basil, tomatoes, and watercress; top with remaining bread. Serve.