Fry the sliced onion and celery in a large, deep pan with a knob of butter and a good glug of olive oil, until the onion is starting to soften.

Add the leeks and potato, pour in the stock, cover the pan with the lid and simmer on a low heat for about 25-30 minutes until everything is very soft. Puree the soup with a stick blender or in a food processor until smooth, adding a little boiled water to thin if necessary. Add salt and black pepper to taste.

To make the crispy onions, heat some light olive oil or vegetable oil in a large frying pan and add the sliced onion, fry, stirring, until just starting to brown and crisp up. Add the spices and salt and pepper, and fry for another minute or two, then add a knob of butter and stir it in as it melts. Remove from the heat.