Gently fry the onion in a little veg oil until soft. Add curry paste and chilli flakes and fry for 2 minutes. Add carrots, lentils, coconut milk and stock. Season with salt and pepper, then simmer for 15 minutes.
Process the soup until smooth then taste for seasoning. Transfer to a clean pan and stir in the rice. Simmer for 10 minutes stirring now and then.
Ladle into bowls or mugs, scatter a few fresh coriander (cillantro) leaves over the top.