Remove tenderloin from turkey; cut tender in 1/2 lengthwise. Cut breast into 3/4-inch thick medallions. Cut open 2 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. Heat 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. 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.
Combine oil, rosemary, thyme, garlic, lemon juice, salt, and pepper.
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