We had this for dinner last night and oh my... it was good.
I've tried 3 or 4 recipes for French Onion Soup now, and this one tops them all (and it only takes 30 minutes!):
Robin Miller's Classic Onion Soup
You can easily substitute another kind of cheese (and it can be sliced or shredded) for the Gruyere and it turns out just as well. I wouldn't suggest substituting the Parmesan rind in the soup, though. It adds a lot.