To make the soup:

Melt the butter in a pan over a low heat on the hob, taking care not to let it burn.

Add the onions and sugar.

Cook stirring at regular intervals until the onions have caramelised and become sticky. This will take at least 30 minutes (see cook 's note).

Once the onions are nicely browned stir in the cider vinegar and allow it to evaporate.

Remove from the heat and stir in the flour followed by the stock, black pepper and the bay leaf.

Return to the heat and continue to cook stirring until slightly thickened.

Place the lid on and simmer over a low heat for 15 minutes to infuse the flavours.

To make the croûtons:

Oven: 200°C/400°F/Gas Mark 6, pre-heated. Fan oven: 180°C. Grill: medium heat setting.

Brush the melted butter on both sides of the French bread pieces.

Place on a baking tray and cook either in the oven, or under a medium heat grill, turning once until lightly browned.

To finish the dish:

Pour the hot soup equally between the two casseroles and arrange 3 croÛtes on top of each.

Sprinkle the grated Gruyère over the top of the croÛtes.

Place either in a hot oven or under the grill until the cheese is melted and bubbling.

Serve with freshly chopped parsley on the top.