Heat the oil and butter in a large frying pan over a medium-high heat. Add the onions with a pinch of salt and fry for 5 minutes. Add the garlic and fry, stirring regularly, until everything is deep golden (25-30 minutes). Bring a kettle of water to the boil.
Scoop out ⅓ of the onion mixture and set aside. Stir the turmeric and cumin into the pan and fry for a minute, then stir in the butter beans. Add the stock pot and 300ml just-boiled water, stirring to help it dissolve. Simmer for 5 minutes.
Stir in the yogurt and chicken and heat through for 2 minutes or until piping hot throughout. Taste and season, if needed, then serve with the reserved caramelised onions and garlic, plus extra yogurt, coriander leaves and some rice or atbreads, if liked.
