Onions became the focal point of my tinkering with other cold-weather classics, including croque monsieur, baked stuffed onions and a savory tart.
The soup has three make-or-break components: broth, onions, cheese. An excellent rendition is layered with flavor and nuance. It starts with an excellent stock; without it, your soup will be average, at best. That means the store-bought stuff in cartons is out.
But stock on its own doesn’t guarantee success. To be honest, my first attempt was surprisingly weak even though I used homemade veal stock. Essentially, it’s best to make a stock from your stock; that is, bump it up by simmering it for 35 to 45 minutes with several skin-on onions (to deepen the stock’s color), celery with leaves (packed with flavor), thyme, black peppercorns and bay leaves. Don’t use carrots; caramelized onions will provide the final product with enough sweetness.
Having run out of veal stock, I used turkey stock and realized that any hearty meat stock (chicken, duck, beef) works in this soup as a fine foil for the onions. The best result, though, came to me by coincidence. I had smoked a couple of chickens for dinner the night before and had filled the drip pan underneath them with aromatics, the chicken’s neck and innards, and herbs. That yielded a rich, smoky stock that went into my soup because it was there; it really made the caramel quality of the onions pop and added a layer of flavor I hadn’t thought of before.
By the way, I’m not against using some bouillon cubes or onion powder or salt to add flavor, or even Kitchen Bouquet to deepen the color of a broth that looks wishy-washy.
Experimenting with yellow, white and Vidalia onions, I discovered that, yes, the Vidalias were sweetest; in fact, too much so to use on their own. I discerned little difference between the yellow and white onions. The soup versions with all yellow onions and half yellow/half Vidalia both had a fine balance of sweet to savory.
You will notice that when you cook onions, they become sweet and remain acrid at the same time, and their odor lingers for quite a while. That is because they contain a good amount of sulfur. When you cut onions, cells are crushed, releasing the sulfuric gas that induces tears that burn. I have no special trick for avoiding the problem, although I certainly have tried more than a few. Just power through the process — and use a food processor for slicing.
The goal in caramelizing onions is to get some color, and therefore flavor, on them quickly and cook them long enough to get rid of their water (10 cups of raw will reduce to barely two cups cooked) and deepen their flavor.