Heat the oil in a large saucepan over medium heat. Add the onion, celery, carrots, sage, salt and pepper. Cook for 5 minutes or until the vegetables are almost tender, stirring occasionally.
Add 5 cups of the broth and the rice. Bring to a simmer and cook for 5 minutes, stirring occasionally. When the rice is tender, whisk the flour into the remaining 1 cup of broth and stir into the soup.
Simmer for 1 minute. Add the chicken and sour cream. Cook on low heat until hot. Garnish with cracked black pepper, if desired.