Put the milk and water in a saucepan and bring to a boil. Gradually add the semolina and stir constantly. Add the butter and continue to cook stirring over low heat for 3-4 minutes until thickened.
In a separate bowl, mix the eggs, ricotta, vanilla, sugar, lemon zest and juice.
Add semolina mixture to the bowl with the eggs and mix well. It is normal to have small lumps.
Pour the mixture in a greased round 22cm baking tin and cook for 40 minutes in a preheated oven at 180°C or until golden on top. Sprinkle with some icing sugar and serve.