Put the rice in a heavy-based saucepan with the milk, cardamom, lemon rind and sugar. Scrape in the seeds from the vanilla pod and place over a medium heat. Bring to the boil, then reduce the heat, partially cover and simmer very gently, stirring regularly, for 25–30 minutes, or until the rice is tender and creamy, adding more milk if necessary.
Meanwhile, place the nuts in a small freezer bag and tap lightly with a rolling pin until they are crushed but not ground. Tip into a nonstick frying pan and dry-fry over a low heat for 5–6 minutes, stirring continuously, until golden. Tip on to a plate and leave to cool.
Spoon the rice pudding into deep bowls and sprinkle over a little extra brown sugar, if liked. Scatter over the toasted nuts and serve immediately.