80g butter, plus a little extra for the toasts
6 onions, peeled and thinly sliced
1 tbsp plain flour
3 sprigs thyme, leaves picked
1 tbsp balsamic vinegar
400ml medium cider
600ml good-quality beef stock
Dash of calvados or other brandy
8 slices of baguette
1 clove of garlic, halved
100g Gruyère, grated
1. Melt the butter in a large, heavy bottomed pan over a low heat. Add the onions, season and cook, stirring regularly, until caramelized and deep brown. (Once they’ve softened, you can turn up the heat a little, but keep an eye on them.) This will probably take between 90 minutes to 2 hours, depending on your nerve.
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. Whisk in the rest of the cider and the stock, and bring to the boil. Simmer for about an hour. Meanwhile, heat the grill and rub the baguette slices with the cut side of the garlic clove. Brush with melted butter, and toast on both sides.
3. Add the brandy to the soup and check the seasoning. To serve, ladle into ovenproof dishes and top with 2 croutons and a mound of cheese. Grill until golden, then serve immediately.