Heat the butter and oil in a big pot, then add the onions and a pinch of salt.
Cover the pot and cook the onions for about 18 minutes or until they are golden and tender.
Stir in the bell peppers and curry paste and cook for 5 minutes to mellow out the curry paste flavor.
Stir in the chicken and cook for 2 minutes.
Add the tomatoes, tomato concentrate and 7 fl oz (200ml) of water.
Cover and simmer for 15 minutes, stirring occasionally, until the chicken is cooked.
Stir in the cream, yogurt and mango chutney and warm the mixture through.
Add salt and/or black pepper to taste if necessary, then scatter with cilantro leaves and serve with naan bread and/or basmati rice.