Since the broth of French onion soup is not high in fat, low-fat recipes typically utilize low-fat cheese or bread. However, another way to minimize the fat content is to add more vegetables to the broth.
Standard French onion soup starts with sliced onions seasoned with salt and pepper and caramelized in a little butter. Sprinkle in flour, and add beef broth plus any flavorings. Typical flavorings include herbs, Worcestershire sauce, balsamic vinegar, minced garlic or red wine. Simmer until thickened. The steps after caramelizing the onions can be done in a crock pot, cooking the broth on low for six to eight hours.
To minimize the fat content or calorie count, recipes call for replacing regular Swiss cheese with a low-fat variety or using less of a stronger-tasting cheese, such as Gruyere. Another option is replacing traditional French bread with whole wheat. Either way, pour broth into bowls, place the bread on top of each portion, sprinkle with cheese, and broil.
The broth can be changed, too. One recipe calls for using three types of onions -- yellow onions, shallots and a leek. There is no difference in how the three types of onions are caramelized. Another option for making French onion soup light is adding 1/2 a bunch of roughly chopped Swiss chard and replacing the beef broth with vegetable broth. Add the Swiss chard after caramelizing the onions, sautéing until the chard wilts. The rest of the steps stay the same.