This beautiful soup originated in France in the 18th century, and it is not just a soup but a way of life. Throughout history, it was seen as humble food for the peasant, as onions grew quickly and plentifully, and broth was made.
Preheat the oven to 400º. Spread the rounds of baguette on a cookie sheet and bake until golden brown. Set aside.
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.
Add the sugar, pinch of salt and increase the heat to high. Cook for about 2 minutes until the onions caramelize slightly.
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.
Remove the oxtails and the thyme. Add the Cognac to the soup. Stir well. Taste and adjust the seasoning.
Turn on the broiler. Leave the door open while reaching the temperature.
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.
Put the soup terrine under the broiler until the cheese melts.
Serve immediately.
