High-sided pan for baking (e.g., a roasting pan or ovenproof glass dish)
Preheat oven to 400°F (204°C). Line the baking sheet with foil and spray with cooking spray. Place chicken breast onto baking sheet and cook until chicken is cooked through, about 45–55 minutes.
While chicken is roasting, make the sauce: Heat olive oil in skillet over medium heat. Sauté onions and garlic until soft, about 4–5 minutes. Add diced tomatoes and cook until the sauce begins to thicken, about 10 minutes. Turn heat off.
While sauce is cooking, cut the stems off of the zucchini and then cut them in half lengthwise. Using a teaspoon, scoop out the seeds and half of the flesh to make a boat shape. Place zucchini skin side down into a high-sided, ovenproof pan that has been lined with foil and sprayed with cooking spray.
When chicken is cooked through, cut it into small pieces and stir it into the tomato sauce. Add the basil. Fill each zucchini with about 2 tablespoons of the chicken mixture. Turn oven down to 375°F (190°C). Cover the entire dish with foil and bake zucchini for 25 minutes.
Uncover the zucchini and distribute the mozzarella cheese evenly on each zucchini boat. Put the pan back into the oven for another 2–3 minutes or until cheese is melted. Serve.