In a large saucepan Combine butter and oil . When butter has melted add in onions, cover and cook over low heat for 10 mins. Blend in salt & sugar , raise heat to medium and let brown , stirring frequently. When onions are golden brown sprinkle in flour and cook slowly for 3-4 mins. Whisk in stock and brandy. Bring to a simmer , cover and simmer for 1 hour.
To serve ladle soup into oven proof crocks, top with a slice of toasted baguette, cover with shredded cheese and broil until cheese is melted .