Heat the oil in a deep frying pan over a moderate heat. Add the butter and cook the onion gently for 8–10 minutes until lightly golden.
Add the capsicum and garlic, sprinkle with the curry powder and mix well. Cook for 1 minute longer. Stir in the lentils and add 700 ml of cold water. Bring to the boil, then reduce the heat, cover and cook gently for 20–30 minutes until the lentils are almost tender but not mushy, and the water has been absorbed.
Stir in the coconut milk and simmer uncovered for a further 5–10 minutes. Season to taste and add the chopped coriander, if you like. Transfer to a serving dish and garnish with with extra coriander. Serve hot with rice and/or flatbread.