From America's Test Kitchen Season 5: Bistro Classics
Streamlined versions of onion soup often amount to a sad crock of flavorless onions floating in super-salty beef bouillon topped with an oily blob of cheese. Or it might be just the opposite: a weak and watery affair. We wanted to make a better onion soup—a dark, rich broth, intensely flavored by an abundance of seriously cooked onions, covered by a broth-soaked crouton with a cheesy, crusty top—and we wanted to make it in record time.
We cheated from the get-go and created a simple broth with store-bought beef and chicken broths and wine. Red onions were chosen for their subtle complexity and nuance, and for the maximum flavor they offered when caramelized. Parsley, thyme, and a bay leaf rounded out the flavors and imparted freshness. To prevent the croutons from getting too soggy, we placed them atop the soup, so only the bottom was submerged. For the cheese, we liked a combination of pungent Swiss cheese topped with subdued Asiago; their flavors added some punch to our speedy but still irresistible soup.
Tie the parsley and thyme sprigs together with kitchen twine so they will be easy to retrieve from the soup pot. Slicing the baguette on the bias will yield slices shaped to fill the mouths of the bowls.