Heat the oil in the base of a saucepan, add the onion and fry until pale golden. Stir in the garlic and spices and cook for 1 minute, then mix in the tomatoes, 600 ml (1 pint) of the stock and sugar. Stir in the lentils, rice, chicken and seasoning, and bring to the boil. Cover and simmer gently for 30–35 minutes until the rice and lentils are tender, topping up with the remaining stock as needed.
Spoon the pilaff into bowls, top with torn herbs, then sprinkle with the pistachios.