Sauté mushrooms in butter over medium heat until golden brown, about 10 minutes. Remove and reserve.
Sauté onion, celery and carrot in olive oil until tender, about 7 minutes. Add in garlic and sage and sauté an additional minute.
Add in rice and stir until coated. Add chicken broth and bring to a boil, then reduce heat to low and simmer covered 15 minutes or until rice is tender. Add in chicken and reserved mushrooms and heat through. Season with salt & pepper to taste.