Onion Soup - Traditional French

0 Preheat the oven to 400º. Spread the rounds of baguette on a cookie sheet and bake until golden brown. Set aside.
1 Heat the butter with the oil in a large saucepan until melted. Add the onions and cook gently over medium heat and occasionally stir. When the onions are completely soft and slightly light golden.  
7 Add the sugar, pinch of salt and increase the heat to high. Cook for about 2 minutes until the onions caramelize slightly. 
6 Stir in flour and cook for about 1 more minute until light brown. Pour in the beef stock, the oxtails, red wine, fresh thyme, add pepper to taste, and bring to a boil. Lower the heat, partially cover and simmer for about 50 minutes.
8 Remove the oxtails and the thyme. Add the Cognac to the soup. Stir well. Taste and adjust the seasoning.
9 Turn on the broiler. Leave the door open while reaching the temperature.
3 Ladle the soup in 4 French onion soup terrines. Cover with the baguette and pile on the Gruyere cheese. Place the terrines on a cookie sheet.
4 Put the soup terrine under the broiler until the cheese melts.
10 Serve immediately.