Finely slice the leek, chop the garlic & celery. Dice the carrot and parsnip.
Heat 2 Tbsp oil in a large saucepan over a medium heat. Add the leeks and celery and sauté for 5 minutes.
Add the garlic, carrot, celery, parsnips and herb mixture then stir for 5 minutes.
Add the lentils, tomato purée and chilli flakes, stir for a few seconds, then add the stock. Bring to the boil, cover and simmer over a low heat for 25-30 minutes, or until the lentils have softened. Stir in the cabbage and cook for 5 minutes. Taste and season, if needed.
Serve in bowls with a squeeze of lemon juice.