To make onion soup, cook 4 sliced onions in 2 tablespoons of butter over medium-high heat until the onions brown, and add beef broth, thyme, salt and pepper to the onions. Simmer the soup for 15 minutes, and serve it topped with bread and melted cheese.
To make caramelized French onion soup, saute sliced onions in butter until the onions are browned, and add 1 to 2 teaspoons of sugar and 1 teaspoon of Worcestershire sauce to the onions. Add 1 1/3 cups of beef broth and 1 tablespoon of water to the onions, and cook for 15 minutes on medium-high heat. Pour the onion soup into oven-safe bowls, top the soup with a slice of toasted French bread and Swiss cheese, and broil the soup until the cheese melts.
To make vegan onion soup, cook 4 onions over medium heat until the onions are browned, and add 2 tablespoons of red wine vinegar to the onions. Stir the mixture, and add 6 sliced cloves of garlic, 1/2 teaspoon of dried thyme, 2 bay leaves, salt, pepper and 6 cups of water to the pan. Simmer the soup for 40 minutes, remove the bay leaves, and top with vegan cheese and a sliced baguette. Broil the soup in oven-safe bowls to melt the cheese.