Clean mushrooms and remove stems. Toss mushrooms caps with olive oil and set aside. Chop mushrooms stems and onion and saute in pan with 1/2 the butter until stems are cooked and onions are opaque. Place dry stuffing mix in large bowl, add cook vegetables. In same saute pan, melt remaining butter and cook sausage until browned. Add to bowl.
Season with salt and pepper to taste. Gradually add chicken broth to bowl until stuffing in well moistened. Fill each mushroom cap generously and place on baking sheet. Bake at 350 for 30 minutes or until well browned.