1. Melt the butter in a large soup pot over low heat. Add the onions, season and cook, stirring regularly, until nut-brown & soft. 90min-120min.
2. Stir in the flour and thyme and cook for a couple of minutes, stirring, then add the vinegar and a third of the cider, stirring and scraping all the brown bits from the bottom of the pan.
3. Whisk in the rest of the cider and the stock, and bring to the boil. Simmer for about an hour.
4. Add the Calvados to the soup and season to taste.
5. Rub the baguette slices with the cut side of a garlic clove and toast on both sides.
To serve, ladle hot soup into ovenproof dishes and top each with 2 croutons and a mound of cheese. Grill until golden, then serve immediately.
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