I don’t have a picture of it: It’s in my head.
There are a couple of secrets to onion soup that I think are worth hammering home. You can fudge your way through it otherwise, but there a two things you can’t skimp on. Wine and cheese.
When you buy your onions, buy the sweetest there are. Not purple. Slice them thinly and caramelize in butter, but don’t sear the edges. Oh, maybe a little. When they’re good and jellified, deglaze with a quarter-cupful of very drinkable white wine. (For a twist, you might even deglaze with an ale, but make it a good one, too.) Stir up the sticky bits and wait for the liquid to all but evaporate.
You don’t have to use home-made stock for a foundation, but if you don’t, mind the salt. Stir the onions into a batch of stock, enough to keep them floating. Cook it for a bit to let it mingle and share skin. Now’s a good time to toss in some thyme — only fresh leaves — if you have them growing out back, or chilling in the fridge.
Assuming you’ve made your crouton (from a baguette, oven-crisped and salt-and-peppered, now lying wait in the bottom of a soup bowl), grate your cheese. I recommend the following: a cave-aged Gruyere. One, it’s available, and not terribly precious. Two, it’s glorious, rich and fruity, a perfect balance to the sweet onion. (Number two choice: an aged Gouda. And I mean aged, hard to the touch, not doughy.)
Some like to stir a bit of flour into the onions as they near the end of cooking, before the wine. That’s old school. Let the crouton manage this end. There is a risk you’ll add too much flour and get a pasty taste. Awful.
To the crouton in the bowl, add enough onion to nearly fill, then wet with enough stock to make the onions swim, then grate onto them a liberal batch of cheese. When you broil, look for coloring, but only slightly. Unless you like a well-burnt cheese, which is no horrible thing. I’d pull it either way, though, if it starts to bubble and squeak.
To drink: More of the white, if you have ample. In fact, make sure you do. A good red is in order, but something about the white — a dry one — makes this soup come to life.