In a large, heavy bottomed saucepan cook the onions in the butter and oil over a low heat for 15 minutes with the lid on, stirring occasionally.
Remove the lid, and turn the heat up slightly to a moderate temperature, add the salt and pinch of sugar, and stir the onions frequently for 30 minutes until they are a delicious shade of dark, almost caramelised brown and melting into each other.
Add the flour and stir quickly for a couple of minutes until evenly combined.
Take the pan off the heat and add the wine, stirring it through, followed by the stock and tablespoon of balsamic vinegar. Return it to a low heat and simmer for roughly one hour, with the lid off. You may want to skim the top off occasionally if a skin develops, depending on preference.
About 15 minutes before you’re ready to serve your soup, make your sourdough toastie croutons. Preheat the oven to 200°C. Place your sourdough slices on a baking tray and cook for 5-10 minutes (depending on how crispy you want them). Remove, top liberally with grated Gruyère, and place back in the oven or under the grill for 2-5 minutes until the cheese has melted through. Add a dash of Worcestershire sauce onto the melted cheese if you like.
When you are ready to serve the soup, add a dash of cognac and stir through immediately before consumption. This soup can also be made in advance and reheated, so if you’re not going to serve the soup immediately save this step until the last minute. Place a sourdough toastie crouton in each bowl of soup and enjoy.
