Saute onions, celery, leek, carrots, and butter in a large pot over medium heat until they start to soften, about 8-10 minutes.Add the flour and the curry, and cook over medium heat for 5 more minutes.Add the chicken stock, mix well, and bring to a boil. Simmer for about 30 minutes.Add the apple, rice, chicken, salt, pepper, nutmeg and thyme. Simmer for 15 minutes, or until the rice is done.When serving, add heated cream (optionally) and chopped fresh parsley.