Peel and quarter the onions, then chop fairly small. Crush the coriander seeds finely, using a pestle and mortar. Heat the olive oil in a flameproof dish or wide pan over a medium heat. Add the onions and crushed coriander seeds and stir together for a minute or two. Now add the orange juice, apple juice and tomato purée.
Cover the dish and simmer very gently over the lowest possible he