Fry the bacon in a large pan over a medium heat for 5 minutes. Add the potatoes, leeks, stock, marjoram, thyme and nutmeg. Cover and simmer, stirring occasionally, for 20 minutes. Season with salt and freshly ground black pepper.
Meanwhile, in a small pan, cook the onion in the butter over a medium heat for 5 minutes. Add the sugar and allow the onion to caramelize, this may take a few minutes.
In a food processor, puree the bacon and vegetables, in batches, until smooth. Return the soup to the pan, and stir in 150ml boiling water.
Ladle the soup into bowls, top each with grated Emmental, some caramelised onions and serve with warm crusty bread.
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