Heat the oil in a frying pan, add the onion and cook over a medium heat for 3 minutes until softened. Add the rice and cook, stirring, for 2 minutes.
Pour in all the stock and bring to the boil, then reduce the heat, cover and simmer very gently for 15 minutes until the rice is tender, stirring occasionally and adding a little more water if necessary.
Add the Parmesan, peas and mint and cook for a further 3 minutes until the peas have defrosted. Stir in the shredded lettuce and stir well, then season well. Serve with warm crusty bread.