Soak the butter beans overnight and then rinse until the water runs clear.
Chop the chorizo into chunks.
Add some oil to a pan and gently fry the chorizo for a couple of minutes until it starts to brown.
Remove from pan and set aside.
Peel and chop the onion and garlic.
Remove the seeds from the pepper and chop into small pieces.
Add the onion, garlic and pepper to the oil and cook gently until soft.
Add the chorizo, stock, rosemary and tin of tomatoes to the pan and bring to the boil.
Simmer, partly covered for an hour until the sauce is thickened and the beans are soft