Heat the oil in a saucepan, add the onion and carrot and fry for 5 minutes, stirring until softened and just turning golden around the edges. Stir in the apple, garlic, tomatoes and curry paste and cook for 2 minutes.
Stir in the sultanas, lentils and stock. Season with salt and pepper and bring to the boil, cover and simmer for 1 hour until the lentils are soft. Mash the soup to make a coarse purée. Add the cooked chicken, heat thoroughly then taste and adjust the seasoning if needed. Ladle into bowls and garnish with coriander sprigs. Serve with warm naan bread or poppadums.