Gruyère cheese is the key to getting the bubbling crust because it melts easily. You can also use blue cheese, comté or aged cheddar. Sometimes I add a splash of sherry or a cup of stout to the stock to bring different flavours.
100g salted butter
8 onions, thinly sliced
1 tsp dark soft brown sugar
2 garlic cloves, crushed
1 tsp dried oregano or freshly chopped thyme or rosemary leaves, plus extra thyme leaves to garnish (optional)
900ml good quality beef stock
sea salt and freshly ground black pepper
FOR THE GRUYÈRE TOASTS
100g gruyère cheese, grated
4 slices of sourdough bread
1. Place a heavy-based saucepan or an ovenproof casserole dish over a medium heat and add the butter. Once the butter has melted, reduce the heat to low and stir in the onions. Cover and cook for 10 minutes until soft.
2. Stir in the brown sugar and garlic and let the onions brown and slightly caramelise, uncovered, stirring every few minutes until they become a deep golden colour. Stir in the dried oregano or fresh thyme or rosemary and season with salt and pepper. Pour in the beef stock and cook for a further 30 minutes.
3. To make the gruyère toasts, preheat the grill to high. Sprinkle the cheese over the slices of sourdough bread and pop under the grill to melt.
4. Serve the onion soup in heated bowls, garnished with a sprinkle of thyme leaves, if you wish, and topped with the gruyère toasts.
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