Preheat the oven to 200°C (180°C fan) one hour before serving.

Combine the stock, bay leaves and thyme in a large pan and heat. Combine the butter and oil in a large, deep frying pan and heat until the butter melts. Add the onions and cook over moderate heat for about 10 minutes, until they are golden and soft. Sprinkle with a little salt, the sugar and flour, and cook for a further 5 minutes, stirring constantly, until onions and flour are turning brown, without burning the onions. Stir in the wine, then transfer the onion mixture to the pan with the stock, and simmer for 30 minutes.

Check seasoning.

Place the bread slices in one layer on a baking sheet and bake them until brown and crisp on both sides. Sprinkle a little of the grated cheese on one side and bake for 1 minute. Turn the oven down to 180°C (160°C fan).

Line the bottom of an ovenproof tureen or individual ovenproof soup bowls with the slivered cheese. Pour the soup over, cover with the toasted bread and sprinkle with the remaining grated cheese. Bake for 20 minutes, then serve immediately.