Preheat oven to 375*. Bring 2 2/3 c. lightly salted water to a boil in a covered medium-sized saucepan. Add the rice, cover and reduce the heat to low. Simmer until the rice is tender, about 20 minutes. Defrost the chicken.
Heat the oil in a 10" or large skillet over medium heat. Peel and coarsely chop the onion, adding it to the skillet as you chop. Continue to cook, stirring occasionally, while you cut the chicken into bite-sized chunks. Add the chicken to the skillet as you cut and cook stirring frequently, until it turns white on the outside, about 4 minutes.
While the chicken cooks, cut the celery into bite-sized pieces and set aside. Add the celery to the skillet and cook, stirring occasionally, until tender, about 2 minutes. Remove the skillet from the heat and set aside.
Spray a 13"x9" glass or ceramic baking dish with cooking oil spray. Pour the cream of mushroom soup and the milk into the dish and stir gently until they are thoroughly mixed. Add the chicken-veggie mix and sprinkle with black pepper, garlic powder, and basil. Stir until the mixture is well combined. Set aside.
Place the butter in microwave safe bowl. Melt butter. Place the cornflakes in a 1 pint or large resealable plastic bag. Seal the bag and crush cornflakes and cover with the butter. Set aside.
When the rice is done, stir it into the chicken mixture in the baking dish, making sure the rice is evenly coated with sauce. Use the back of the spoon, spread the mixture evenly in the dish. Sprinkle the buttered cornflakes evenly over the ingredients. Bake uncovered until the mixture is bubbling at the edges and the top is lightly browned, about 30 minutes.