Arrange chicken in a shallow roasting pan. In a medium bowl, combine onion, garlic, olive oil, lemon juice, Italian seasoning, salt, black pepper, and crushed red pepper. Arrange the onion and garlic pieces around chicken; brush the oil mixture over chicken.
Bake in a 375 degree F oven for 25 minutes, stirring onion once. Add zucchini and bake for 8 minutes. Add tomatoes, stir, and bake about 5 minutes more or until chicken is no longer pink (170 degrees F) and vegetables are tender.
Transfer chicken to a serving platter. Add romaine and vinegar to pan. Return pan to oven; bake about 5 minutes or until romaine just begins to wilt, stirring and tossing occasionally. Serve hot salad immediately with chicken. Makes 4 servings.