Here's the basics.
Slice up 4 or 5 onions into slices, and then saute them in some olive oil and butter over fairly low heat for about 45 minutes, stirring from time to time.
The onions should be nice and translucent, and starting to turn golden brown. Then pour one cup of water over the onions, stirring them to release any fond left on the pan, you do want all that luscious browned bits to be incorporated into the soup. At this point you can either make up the Au Jus with 5 cups of water in a separate pot or pour the Au Jus mix over the onions, put a lid on it and let them simmer for about 30 minutes, until the onions are tender.
I add about 1-2 tbsp. of Worcestershire sauce the last few minutes while the soup is simmering. I've found if I add the Worcestershire sauce too early, it can develop a bitter taste.
While the soup is simmering, prepare your cheese toast.
If you have some nice beef stock in the freezer or cupboard, by all means use that, but I rarely, if ever, have it, but I do have those wonderful little packets of Au Jus mix in my spice drawer. They're great to have on hand for quick seasonings, soup, mock stock, etc.