Fill a kettle and bring to the boil. Meanwhile, place a saucepan over a high heat. Add the tomatoes, garlic, harissa and stock cube, to soften.
Drain the kidney beans and chickpeas and add to the pan, then stir well. When the kettle has boiled, measure out 1 litre (1¾pt) of water and add to the pan. Remove pan from the heat and blend mixture until smooth.
Return the pan to the heat and cook for a further 3 minutes. Meanwhile, chop the coriander. Stir the couscous into the soup and cook for one minute before adding the coriander and lemon juice. Season to taste.