In a food processor, whizz the onion, celery and red pepper to a purée. Heat the oil in a large pan and cook the purée for around 5 minutes.
Stir in the tomato purée and sugar. Add the tomatoes and lentils to the pan. Make up 500ml stock using the chicken stock cube and add to the pan too. Bring to the boil, then simmer with the lid on for a further 30 minutes.
Uncover and tip in the chickpeas, cook for a further 5 minutes and top up with water if necessary. Season to taste. Stir through the lemon juice and chopped coriander just before serving.
Cook's tip: for a little kick, why not add a pinch of chilli flakes?