1. Melt the butter in a large frying pan.
2. Saute (on fairly high heat) mushrooms, onion and water chestnuts until mushrooms are browned and moisture has evaporated from the pan.
3. Add drained green beans.
4. In a separate bowl, combine soup, half-and-half, and seasonings.
5. Pour soup mixture over veggie mixture and place in a 9x13 baking dish.
6. Bake at 375 degrees for 30 minutes.
7. Top casserole with onion rings and continue to bake an additional 10 minutes, just until onion rings begin to brown a bit.
8. Serve immediately.