From America's Test Kitchen Season 9: French Classics, Reimagined
With too many onion soups, digging through a layer of congealed cheese unearths a disappointing broth that just doesn’t taste like onions. The ideal French onion soup combines a satisfying broth redolent of sweet caramelized onions with a slice of toasted baguette and melted cheese. We wanted a foolproof method for achieving extraordinarily deep flavor from the humble onion—the star of this classic soup.
The secret to a rich broth was to caramelize the onions fully. The good news is that caramelizing the onions, deglazing the pot, and then repeating this process dozens of times will keep ratcheting up the flavor. The bad news is what a laborious, hands-on process this proved to be. Fortunately, we found that if we first cooked the onions covered in a hot oven for two and a half hours, we only needed to deglaze the onions on the stovetop three or four times. Just one type of onion (yellow) was sufficient, but a combination of three different liquids (water, chicken broth, and beef broth) added maximum flavor. For the topping, we toasted the bread before floating it on the soup to ward off sogginess and added only a modest sprinkling of nutty Gruyère so the broth wasn’t overpowered.
Sweet onions, such as Vidalia or Walla Walla, will make this recipe overly sweet. Be patient when caramelizing the onions in step 2; the entire process takes 45 to 60 minutes. Use broiler-safe crocks and keep the rim of the bowls 4 to 5 inches from the heating element to obtain a proper gratinée of melted, bubbly cheese. If using ordinary soup bowls, sprinkle the toasted bread slices with Gruyère and return them to the broiler until the cheese melts, then float them on top of the soup. We prefer Swanson Certified Organic Free Range Chicken Broth and Pacific Beef Broth. For the best flavor, make the soup a day or 2 in advance. Alternatively, the onions can be prepared through step 1, cooled in the pot, and refrigerated for up to 3 days before proceeding with the recipe.