Heat the oil in a large saucepan, add the onion and gently fry until soft. Add the curry paste and chilli flakes and fry for another 2 minutes before adding the grated carrots, lentils, coconut milk and stock. Season, then simmer for 15 minutes, or until the lentils are soft (squash one against the side of the pan to check).
Whizz the soup with a stick blender until smooth, season to taste, then stir in the basmati rice. Simmer for another 10 minutes, stirring now and again, until the rice grains are tender. To serve, ladle the soup into mugs, scatter with a few fresh coriander leaves, and offer some naan on the side.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.