Cook the broad beans in a large pan of boiling salted water for 3-5 minutes or until just tender. Plunge into ice-cold water to cool. Drain, peel off the outer skin, if wished, and set aside.
Melt the butter in a large saucepan, add the onion and cook over a medium heat for 5 minutes or until beginning to soften. Add the rice and continue to cook, stirring for 1-2 minutes.
Pour in a ladleful of the hot stock and simmer gently, stirring frequently until the rice has absorbed most of it. Keep adding the stock I nthis way until the rice is tender but still has bite to it; this will take about 15-20 minutes. The risotto should look creamy and soft when cooked.
Add the broad beans, lemon rind and juice and warm through.
Serve the risotto immediately, garnished with grated Parmesan and lemon rind.