In a large dutch oven, melt butter, add sliced onions, sugar and salt.  Cook for approximately one hour over medium low heat, stirring frequently until onions are caramelized and dark brown.  Add stock, thyme, sherry, salt and pepper.  Simmer on low for 45 additional minutes.  

Preheat broiler.  On a foil lined baking sheet place baguette slices and top with grated cheese.  When ready to serve the soup, broil bread sliced until cheese is melted and edges begin to brown, watch carefully. 

Ladle soup into bowls and top with toasted bread slices, serve.