Place butter and onions into pan and saute until onions are glazed. (about 5-7 minutes).
While onions are sauteing roughly chop a few mushrooms into small pieces.
Add mushrooms to onions.
Let saute for another 5 minutes or so.
Add chicken broth and stir, scrapping the bottom of the pan to loosen cooked on bits from the saute process.
Add a pinch of salt and pepper (more if you like pepper).
Add remaining mushrooms. Cut some into fourths and others into smaller pieces, as small or large as you want. If you have some really small mushrooms you may even choose to leave them whole.
Let simmer on medium or medium low heat until you are ready to eat.
This is great to start and let simmer while you clean, or get some other work done. The longer it cooks the more flavor it will have, but it still taste great even if you just let it cook long enough to soften mushrooms. There really isn't a set simmer time.