- Heat the olive oil and butter in a large pot over medium heat and add the onions and thyme. Season lightly with salt and pepper and cook the onions down, stirring often, until they are caramelized – this takes about 40 minutes. If the onions start sticking to the pan after the first 10 minutes, reduce your heat slightly, but after that point, if the onions stick, add a splash of the sherry to deglaze the pan and distribute the colour and flavour.
- Preheat the oven to 400°F (200°C), and stir in the (remaining, if you’ve used some) sherry and let the onions soak it in. Pour in the beef stock and bring up to a simmer. Let the soup simmer for about 15 minutes and then season to taste.
- Arrange 6 or 8 ovenproof soup dishes on a baking tray and ladle the soup into them. Place a toasted bread slice on top of each and sprinkle the cheese overtop in an even layer. Bake for about 15 minutes, until the cheese bubbles and browns, and the soup is bubbling at the edges. Serve immediately.