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.
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. The answer was to first cook the onions covered in a 400-degree oven for two and a half hours (we also developed a faster modification of this first step using a microwave, although the flavor is not quite as deep). Then we only needed to deglaze the onions on the stovetop three or four times. Our final testing proved that we needed to use just one type of onion (yellow), 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.