Soak butter beans in water for about 12 hours.
Preheat the oven to Gas 6, 200°C, fan180°C.
Drain the beans, put in a roomy pan with fresh water, bring to the boil and simmer for 45 minutes or until cooked.
Roast potatoes with oil and seasoning for 25 minutes until cooked.
Saute the leek, garlic and chorizo for five or so minutes. Add the lentils, beans, tomato puree, rosemary and stock and simmer gently for 20-25 minutes until the lentils are soft but not falling apart.
Add the sweet potatoes, stir everything together well, season and serve.
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