Preheat oven to 350 degrees F. Coat a 9×13 baking dish with cooking spray. Set aside. In a Dutch oven (or heavy saucepan or skillet), heat the olive oil over medium heat. Add the onions, celery and carrots and cook, stirring, until softened, about 10 minutes. Add garlic and cook for 1 additional minute. Add the chicken, wild rice, brown rice, salt and pepper. Mix well and remove from heat.