Heat half the butter in a saucepan and fry the onion until softened. Add the rice and stir for about 4 minutes. Add the stock, ladle by ladle, stirring over a low heat until all the water is absorbed. Add the saffron and its soaking water. It should take around 20 minutes for all the water to be absorbed and for the risotto to be creamy and moist.
Now stir in the remaining butter and th