Gather the ingredients.
In a large skillet or saute pan over medium-low heat, melt the butter. Sauté the sliced onions for about 30 to 45 minutes, or until very soft and golden brown in color. Stir them frequently. If they are browning too quickly or scorching, turn the heat to low. It takes time to get that sweet caramelization, so don't try to rush it.
In a medium saucepan or Dutch oven, combine the beef broth with the cooked onion and bring to a boil. Reduce the heat to low and cover the pan. Simmer for 25 to 30 minutes.
Meanwhile, heat the broiler and place the French bread slices on a foil-lined baking pan. Toast the French bread about 4 inches from the heat until golden brown on both sides.
Sprinkle each slice evenly with the Gruyere cheese. Return to broiler and broil just until cheese is melted and bubbling.
To serve, pour the onion soup into four individual soup bowls. Float a slice of toasted French bread, cheese side up, in each bowl and sprinkle with extra cheese.
Serve and enjoy!