Bring 3 1/2 cups of the chicken stock to a boil. Add long grain brown rice and wild rice. Cover, lower to a simmer, and cook for 40 minutes, stirring periodically to check that you still have water. You want your rice to just be tender.
Meanwhile, heat olive oil in a large soup kettle over medium heat. Once ho, add celery, carrots, onions, garlic, salt, pepper, and sage. Cook over medium-high heat for 10 minutes, until tender.
Lower the heat to medium and stir in flour and butter. Continue to stir vegetables, butter, and flour for 1-2 minutes. SLOWLY stir in the milk, 1/2 cup at a time.
Stir in cooked rice and chicken. Then slowly stir in the remaining chicken stock.
Allow to simmer for 20-30 minutes. Serve.