Heat oil in a large saucepan over medium heat. Add the onion, carrot and celery and cook, stirring gently for 5-10 minutes. Add the red lentils, chilli, tomato paste,ground coriander, ground cumin and stir to combine. Cover with stock and bring to the boil. Turn heat down, season to taste, cover saucepan and simmer for 30-40 minutes.