Oven-baked French onion soup with Gruyère toasts

Method

1. Heat the oven to 160°C/140°C fan/gas 3. Divide the onions and garlic equally between 2 medium roasting tins (about 2.5 litres capacity each). Add 1 tbsp vinegar, 1 tbsp sugar, half the thyme and 50g butter to each tin, then divide the stock and sherry equally between the tins, too. Cover both tins with a layer of baking paper, then a layer of foil, transfer to the oven and cook for 2½ hours. At the end of the cooking time, transfer the contents of both tins to a heatproof ceramic 2 litre dish (see Make Ahead).
2. Set the grill to high. Place the bread slices on top of the onion soup, then top with the cheese. Put under the hot grill for 5 minutes or until the cheese has melted and turned golden. Serve straightaway.