Toast the coriander seeds in a large pan over a low heat for about 4 minutes until fragrant, then lightly crush in a pestle and mortar. Tip onto a plate and set aside.
Heat the oil in the pan over a medium heat. Add the celery, garlic and chillies and cook for 5 minutes. Stir in the toasted seeds and lentils and cook for a further 2 minutes. Add the coconut milk, stock and 100ml water,