- Place a large soup pot over medium-high heat and melt the oil and butter together. Once that is hot, add the onions, garlic, and sugar. Keep stirring them every so often and cook until they start to darken, about 5 minutes.
- Now, reduce the heat to low and let the onion mixture cook for about 30 minutes, stirring occasionally. The bottom of the pot will develop a dark, caramelized color- don’t worry, it’s not burning! This build-up is where lots of flavor will come from.
- Add the stock, wine, and cognac and stir to incorporate. It is now the time to use a wooden spoon and scrape off all that caramelized goodness from the bottom. Let the soup simmer on low uncovered for about an hour.
- While the soup is doing its thing, get to work on the croutons. On a baking sheet, mix the oil, garlic and salt and spread it around with your fingers. Rub both sides of each slice of bread in the oil mixture. Bake for 20-25 minutes at 180°C/350°F until golden and crunchy.
- Check the flavor of the soup and season accordingly. To serve, ladle it into a bowl and top with a crouton and a good sprinkle of Gruyère. Place the bowl under a hot grill until the cheese is decidedly melty. Then, set the bowl on a plate and serve to your hungry guests, making sure to warn them of the hot bowls!
As a variation- get creative! Mix it up with different types of stock or add your own favorite tipple instead of cognac; even though cognac goes bloomin’ well in this soup.