French Onion Soup (Soupe à l'Oignon Gratinée)

"It's easy to make amazing French onion soup, as long as you pay attention to a few key details. [Photographs: Vicky Wasik unless otherwise noted] For such a simple dish, French onion soup should be easy to make great. And yet so many versions taste like a cup of burnt-onion tea with melted cheese trying its best to cover up the flaws. This recipe develops great flavor with a long, slow caramelization of the onions, then punches it up with cider vinegar, fresh chives, and a surprise ingredient: fish sauce. Why this recipe works: * Caramelizing the onions slowly in butter until rich golden brown (but not so dark as to taste bitter) produces the most flavorful, sweet results. * The homemade or store-bought chicken stock used here are typically better quality than the store-bought beef stock many recipes call for. * Asian fish sauce, cider vinegar, and sherry add depth and complexity to the broth. Note: Yellow onions are your best all-purpose bet for this soup, but a mix of yellow, sweet (e.g. Vidalia), and red onions, as well as shallots, produces and even more complex flavor...."