To make French onion soup in a slow cooker, combine onion, broth and spices. The soup is cooked on low for 10 hours before being placed in serving bowls and topped with French bread and melted cheese.
Spray a slow cooker with cooking spray and add 14 cups of thinly sliced onion, and 1/4 cup of melted butter. Sprinkle in one teaspoon of salt, one teaspoon of pepper and 1/4 teaspoon of dried thyme. In a small mixing bowl, blend 3/4 cup of sherry, five tablespoons of flour and 1/4 cup of water with a whisk until smooth. Add the flour mixture to the slow cooker, cover and cook on low for 10 hours or until the onions are tender and dark brown.
Add four cups of beef broth to the onions in the slow cooker and increase heat to high. Cook on high for 10 minutes or until hot. While the soup is finishing, line a baking pan with foil and place several thick slices of French bread on the pan. Allow one slice of bread per bowl of soup. Place the baking sheet in an oven set on broil and broil for 2 minutes per side or until lightly toasted. Sprinkle 1 1/2 cups of Gruyere or Mozzarella cheese over the toasted bread slices and return to broiler for one minute, or until the cheese is melted.
To serve, place soup in serving bowls and top each bowl with a slice of toasted bread.