When we roast a chicken or a turkey I always save the carcass and make stock. We've got lots of chicken and turkey stock in the freezer. I also save the bones from beef roasts, lamb, and pork - brown them up nicely in the oven with carrots, celery, some herbs, and onions and make stock from them. It's easy to keep a supply of stock on hand if you just stash away bones, etc.
VeraBlue is right on - must have thyme in French Onion soup. I usually use chicken stock and white wine in mine. I also like a combination of onions, white, yellow, red, leeks, and shallots. Sometimes garlic. A trick I use is to toast some peppercorns and grind them into the soup. There's a real pop of flavor there. Hmmmmmmm- do I have enough onions???? This is sounding really good for dinner tonight.