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.