A simmering pot of French onion soup fills your kitchen with fragrant aromas from the onions, herbs and broth, and pleases your palate with a flavor combination that is at once bold and sweet. Don't let the long simmering time intimidate you. A basic French onion soup uses only a handful of ingredients and is fairly simply to prepare.
Sweet onions are an absolute must in French onion soup, but you might try multiple types of onions for a broader flavor profile, such as yellow onions or red onions. Cut the onions into very thin slices -- try a mandolin slicer to achieve uniform thickness. Use about 1 cup of onions for each quart of finished soup you wish to make. Cook the onions in a few tablespoons of butter for 5 to 10 minutes over medium-high to sweat them and cook off the liquid. Reduce the heat to medium-low for about 45 minutes until they are soft and a deep brown color. The key to caramelizing onions is to leave them in the pan for several minutes between stirring.
Pour beef broth over the onions and scrape the bottom of the pan with a wooden spoon to deglaze the pan and infuse it with the cooked-on onion flavor. You'll need about 2-1/2 cups of broth for a 4-quart pot of soup. Choose from fresh beef broth, canned broth, or make a simple broth by dissolving beef bouillon cubes in water. Basic French onion soup is seasoned with black pepper, but try other spices such as a sprig of thyme and a bay leaf. Beef broth usually contains enough salt, but you might season homemade beef broth with a bit of salt. Bring the broth to a boil, reduce the heat and simmer for 30 minutes to 2 hours.
One of the most distinguishing features of French onion soup are the large croutons or toasted bread served over the onions in the bowl. The bread soaks up the excess broth, becoming soft and smooth. French bread is the most obvious choice because it is dense and hearty. Slice the bread about 1 inch thick, ideally choosing a loaf close to the diameter of the bowl. If you prefer several smaller croutons, slice a baguette into 1-inch slices. Lightly butter each side of the bread and toast for a few minutes until golden brown.
Serve the finished soup in oven-proof bowls, dividing the onions and broth equally among each bowl. Rest a piece of toasted bread on top of the onions. Cover the bread with a slice of cheese, such as Swiss cheese or Gruyere cheese. Look for cheese large enough to completely cover the bowl opening with a slight overlap over the edges. Place the bowls on a baking sheet and place under the broiler for a few minutes until the cheese is melted, bubbling and begins to brown. Serve any extra bread pieces on the side for dipping.