Place a large cast-iron or thick-bottomed saucepan over a low heat. Add the onion, shallot, leek, garlic, butter and olive oil. Slowly sweat until very soft, then caramelise slightly for about 40 minutes.

Add the balsamic vinegar and stir for 1 minute before adding the stock. Simmer for 10 minutes with the bay leaves, then remove from the heat, add the sherry vinegar and season to taste.

Serve warm with French-loaf croutons topped with a generous pile of grated Parmesan cheese.

Cook's note: the stock can make or break this soup. Buy the best quality, freshly made stock you can find. I use Woolworths' liquid organic beef stock.