Sauté the leek and let it consume until it becomes creamy (add a little water). When the leeks are cooked, add the rice.
Boil a pot of water with a vegetable stock cube. Pour occasionally with a ladle the hot broth until the rice is consumed and continue to pour the broth, mix well until the end of the cooking time (20 minutes).
When the rice is cooked add the butter and parmesan, mix well and leave to rest for 5 minutes. If you want, to make it more creamy, add soft cheese or sottiletta at the end.