Heat the oil in a large, heavy-based saucepan. Add the onion and cook for 5 minutes until softened. Stir in the risotto rice, followed by the wine and bubble until reduced by half.
Gradually add the stock, a ladleful at a time, stirring continuously and allowing each ladleful to be absorbed before adding the next. Continue to cook for about 15 minutes, or until the rice is tender.
Stir in the prawns, peas, lemon rind and butter and season to taste. Cover and leave to rest for a few minutes, then add the lemon juice and serve sprinkled with the mint.