Rinse the lentils under cold water then soak overnight (if needed - see packet).
Finely chop the onion, garlic and celery. In a large saucepan, slowly sweat them, over low heat, until soft.
Meanwhile dice the carrot and red pepper. Add these to the pot together with the spices and tomato purée. Combine well and gently sauté for a couple of minutes.
Add the lentils, stock, tinned tomatoes, then bring to the boil. Turn down the heat and gently simmer for 25-30 minutes or until the lentils are cooked. Add more stock, if you find the soup too thick.
Serve the soup with a dash of cider vinegar and some freshly ground pepper, garnished with chopped herbs of your choice.