Wash and pat dry the chicken breasts. Cut them lengthwise into 1-inch strips.
Preheat oven to 375 degrees F. Lightly spray an 11x8-inch baking dish with cooking spray. Spread 1/4 cup basil pesto over the bottom of the baking dish.
Spread the chopped onion. Add the celery. Lay the chicken strips over the onions. Season with salt and pepper.
Spread 1/4 cup basil pesto over the chicken.
Cover the baking dish with aluminum foil and bake in the preheated oven for 40-45 minutes or until the chicken is cooked through.
Remove the aluminum foil and sprinkle the chicken with shredded mozzarella cheese. Put the baking dish back into the oven and bake uncovered until cheese is melted, about 7-8 minutes. Then switch the oven to broil and broil until cheese is golden, about 3-4 minutes.
Discard the celery. Serve hot with salad, bread or pasta.