In making this soup I turned to James Peterson's excellent volume, Glorious French Food (Wiley, hardcover, $45), in which he presents several onion-derived soups, including the one I used as the basis for this soup. Here I've cut the recipe in half and made some minor changes but the credit still goes to Peterson.
With so few basic ingredients, French onion soup is remarkably versatile. You should feel free to experiment and make substitutions as long as you follow the basic procedure. I happened to have goose fat on hand so I used it to sauté the onions but you could also use canola oil, butter or olive oil. Because the recipe calls for so little fat, it's probably safe to choose your fat based on flavor.
Beef stock is probably the most common base for this soup but feel free to choose whichever kind of stock you like, knowing that the dominant flavor will still be onion. It's cheaper to buy a small jar of concentrated bouillon paste than it is to buy stock in cans. Look for one without MSG and with relatively low sodium.
Tears of Pain
When onions are chopped they release sulfuric vapors that irritate the eyes and, to a lesser degree, the nasal passages. There's really only one way to prevent the fumes from making you cry like a little girl: Don't let them reach your eyeballs. Though contact lenses do help, the best method is to wear goggles, whether they're for working with power tools, skiing or swimming. My personal preference is swim goggles simply because they are definitely not vented (as many ski and shop goggles are), therefore blocking virtually all of the evil vapors. Sure, you look like a dork but who cares.
Chop, chop, chopping
Slicing several pounds of onions doesn't actually take that long, assuming you've got your eye-protection in place and a sharp knife in hand. Don't be tempted to use a food processor for this recipe—it won't slice the onions thinly enough. Using a mandoline or a Japanese vegetable slicer would be quick and effective.
Red, White, Yellow?
Peterson recommends using red onions because they are moderately sweet. The very sweet Vidalia, Maui or Walla Walla onions are relatively expensive and their sweetness would likely be wasted on this soup.
While You're At It
You can also use this recipe (stop before adding the stock) simply to make caramelized onions. Use them for homemade pizzas, in sandwiches, on top of pork chops or bruschetta, in quiches and omelets. Divide the total amount into smaller portions and toss them into the freezer for future use.