Heat the oil in a saucepan. Add the onion and cook over a low heat for 7–8 minutes, stirring occasionally until it softens. Stir in the carrot and cook for 3 minutes, stirring frequently. Add the garlic, cumin and coriander and cook for a further minute, stirring constantly.
Add the lentils, stock, tomatoes with their juice, tomato purée. Bring to the boil, reduce the heat, then half-cover the pan with a lid and simmer for 25–30 minutes until the lentils and vegetables are very soft.
Blend the soup in the pan using a hand-held blender, or tip into a blender or food processor, process until smooth, then return the soup to the pan. If the soup is too thick for your taste, dilute it with stock or water.
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