Soak butter beans in water for about 12 hours.
Preheat the oven to Gas Mark 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.
Sauté the leek, garlic and chorizo for five or so minutes. Add the lentils, beans, tomato purée, 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.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.