Fry the onion for at least 5 minutes, over a medium high heat, until softened.
Next add the garlic, chillies, curry leaves and mustard seeds. Keep frying for another couple of minutes.
Sprinkle in the spices, then stir well. Add in the chopped peppers then pour over the chopped tomatoes and stock. Simmer for 10 minutes.
Finally, pop in the butter beans and stir well. Simmer for 20 minutes until the sauce is nice and thick. Serve and enjoy!