In a pot put the milk, water, rice, sugar and salt to boil. Just before the rice is cooked, add the flour as a thickener.
Simmer a bit more to thicken slightly and remove from the heat.
Optionally, add the vanilla, and if you want, lemon zest too.
Finally, you can add the knob of butter. Pour into bowls. Optionally sprinkle it with cinnamon.