Rinse the chicken breasts and pat dry. Place 2 tablespoons lemon juice in a resealable plastic bag. Add the chicken breasts. Seal the bag and refrigerate. Marinate the chicken for 1 hour, turning the bag occasionally so that all the chicken breast meat is covered in the juice. Remove the marinated chicken from the refrigerator and pat dry. Discard the marinade.
Heat the vegetable oil in a frying pan over medium heat. Add the onion and cook until tender. Add the chicken and cook until it turns white, about 5 minutes. Add the zucchini and cook for 1 minute.
Add the chicken broth, tomato sauce, remaining 2 tablespoons lemon juice, and salt and pepper. Cover and simmer for 20 minutes. Serve hot.
When adjusting the portions of a recipe, remember that the quantity of food affects the total cooking time. For example, the cooking time for 4 potatoes will be longer than the cooking time for 1.
Delicately flavored Steamed Jasmine Rice (see page 81) would make an excellent accompaniment to this dish. Prepare the rice while the chicken is simmering.