Combine the rice, milk, saffron and water, vanilla bean, sugar, raisins, salt and cardamom. Mix well and bring to the boil in a saucepan, then cook over very low heat 1-2 hours (use a heat diffuser if you have one). Take care not to let the rice boil, just let it steep until it is thick and creamy.
Just before serving, remove the vanilla bean and stir in the lemon zest. Serve warm or cold.