Put the chickpeas in a bowl, add cold water to cover by 10 cm (4 inches) and leave to soak overnight.
Drain the chickpeas, rinse under running cold water and drain again. Place the chickpeas in a saucepan of cold water and bring to the boil. Boil rapidly for 10 minutes, then reduce the heat and simmer gently, partially covered, for about 1 hour or until tender, adding more water as necessary. Drain well.
Meanwhile, heat the oil in a large, heavy-based saucepan over a medium-low heat, add the garlic, onion, celery and carrots and cook for 15 minutes, or until softened, stirring frequently. Add the spices and cook for 1–2 minutes, then increase the heat and add the chickpeas, lentils and tomato purée. Pour in the stock and bring to the boil, then reduce the heat and simmer gently for 40–45 minutes.
Spoon the cooked pulses and vegetables into serving bowls and carefully pour the liquid around the side. Sprinkle with parsley and serve with harissa, to taste.