Chop equal parts onion, celery, carrots, and mushrooms. (Example: 1 cup each)
Saute in butter until they begin to soften. The mushrooms will release most of their moisture and it needs to cook away. Stir consistently so nothing burns.
Add ample chicken broth.
Bring to a boil and let this simmer for five-ten minutes.
Add cooked brown rice and sliced or shredded chicken breast.
You could add uncooked rice and chopped, raw chicken, but keep the amounts small. For instance, with two quarts of stock, I'd add a cup of uncooked rice and only one chicken breast, chopped. You'll also need to add a little more liquid (water or broth or a combination), since the rice will soak it up. If you are cooking rice and chicken as you go, bring your soup back to a boil and cook for several minutes until the rice and chicken are totally done.
Salt generously and add parsley and black pepper to taste.
Enjoy with crackers.
Freeze leftovers for a rainy day. This is a great soup to thaw and reheat when you are sick and are in need of something lite but nourishing.