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To fry the vegetables, heat the oil in a large saucepan and fry the onion for 5 minutes until golden. Stir in the celery and fry for 5 minutes, then tip in the potatoes and fry for a further 1-2 minutes, stirring often.
Stir in the curry paste, let it cook for a minute or so, then pour in the stock. Bring to the boil and stir well. Lower the heat, cover and simmer for 15-20 minutes until the potatoes are tender.
To serve, tip the leftover veg into the pan and warm through for a few minutes. Pour the soup into a food processor or blender and blitz to a smooth purée. Thin down to the consistency you like with hot water or stock (we added 300ml) then taste for seasoning. Cool and freeze, or serve in bowls with spoonfuls of yogurt or crème fraîche swirled on top.