To finish the risotto (“mantecatura”):
Melt the butter in a heavy based pan and add the chopped onion. Cook gently until translucent but don’t colour the onion (for about 5 minutes).
Now add the rice into the pan and stir for a couple of minutes. All the grains should be coated with the mixture and warm (they will start changing colour).
Next pour in the wine that should cover the rice and let it evaporate completely. Once the onion and rice are dry, add the saffron and start pouring in the stock in small amounts, even a ladleful or two at a time while continuously stirring the risotto base. When each addition of the stock has almost evaporated add another one (again a ladleful or two, never add too much).
Carry on cooking the risotto for 15-17 minutes on a medium heat (which is long enough for the rice to turn al dente) but slow down with adding the stock after 12-15 minutes. You don’t want the risotto to become too runny or wet, it should be creamy instead therefore pay attention on how much stock you are adding towards the end of the cooking process.
Once the grains are ready and soft (al dente) turn the heat down and wait a minute.
The last step is to add the diced cold butter and the grated cheese beating it vigorously together (very important) while shaking the pan. Season to taste.
I like to cover the pan with a lid and let it rest for no more than 2 minutes but you can of course serve the risotto immediately.