The essence of an authentic onion soup au gratin is, well, the onions -- or more specifically, caramelized onions, which are the key ingredient in this classic French country-style soup. Another key "ingredient" is a generous amount of patience.
When onions caramelize, they transform from a crisp, bulbous root into a slippery, squidgy tangle of mahogany ribbons. To achieve this desired sludge, a good amount of time is required, and this is where you must rely on patience. You may find recipes for caramelizing onions in which the onions are fried on the cooktop and browned in a mere half-hour. While they may appear to be caramelized, they are, in fact, not.
To properly caramelize, onions need time to sweat, break down and release their sugars and juices. The juices, in turn, must slowly evaporate, allowing the onions to concentrate their sugars, and brown and caramelize in spots. Then you must scrape up the brown bits in the pan, give the onions a good stir and repeat the process again and again until you are left with a squishy slump of onion, thoroughly burnished a deep golden brown.
It should take at least one hour and may take up to 90 minutes. If you stop too early at the first hint of color, you will simply have fried onions, which will lack that extra depth of sweet richness and caramelization that is the hallmark of French onion soup.
Onion soup au gratin
Active time: about 1½ hours
Total time: about 1½ hours
Yield: Serves four
3 tablespoons unsalted butter
Extra-virgin olive oil
3 pounds yellow onions (about five large), halved and thinly sliced
¼ cup Calvados brandy
½ cup dry white wine
6 cups homemade or good-quality chicken stock
4 thyme sprigs
1 bay leaf
1 tablespoon apple cider vinegar
Freshly ground black pepper
4 thick center-cut slices country-style bread, or 8-12 thick slices baguette
1½ cups coarsely grated Gruyere or Comte cheese, or more as needed
Melt the butter with 1 tablespoon olive oil over medium heat in a large Dutch oven or heavy pot with a lid. Add the onions and 1 teaspoon kosher salt. Cover and cook until soft, seven to eight minutes, stirring occasionally.
Reduce the heat to medium-low. Partially cover the pot and cook, undisturbed, until the onions soften and begin to brown in spots, stirring up the brown bits every 20 minutes or so. After one hour, remove the lid and keep an eye on the onions, occasionally stirring up all the brown bits. When the onions are sludgy and deep golden mahogany brown in color, they are ready, 20-30 minutes more.
Add the brandy to the pan and deglaze, scraping up any browned bits. Add the wine and simmer until the liquid is nearly evaporated. Add the chicken stock, thyme, bay leaf and apple cider vinegar. Bring to a boil, then simmer for 15-20 minutes over medium-low heat. Season with salt and pepper.
While the soup simmers, toast the bread. Heat the oven to 400 degrees. Lightly brush the bread slices with olive oil. Arrange on a baking sheet and bake until light golden and crisp, 8-10 minutes.
Divide the soup evenly among four ovenproof bowls or crocks arranged on a baking sheet. Gently lay slices of bread on top (enough to mostly cover the soup without overlapping) and press down to slightly submerge the bottoms. Sprinkle a generous amount of cheese over the bread. Transfer to the oven and broil until the cheese is bubbling and golden brown, one to two minutes. Remove from oven and serve immediately.
Lynda Balslev is an award-winning writer, cookbook author, and recipe developer based in northern California.