Bring a pot of salted water to a boil. Meanwhile, shell the broad beans and discard the pods. Boil the beans for 4 minutes, strain and then immediately plunge into ice water. Let cool for 2 minutes then pierce the beans and squeeze them out of their skins. Separate 3/4 of the broad beans and puree in a food processor.
In a separate large saucepan bring the stock to a simmer, and keep it hot. Meanwhile, in another large saucepan over medium heat, melt half of the butter and add the onions. Reduce the heat to low and cook for about 5 minutes; do not brown the onions.
Add the rice and cook, while stirring, for 2 minutes making sure the rice is well coated with the butter. Add the wine, increase the heat to medium, and stir constantly. When the wine has been absorbed, add about a cup of the hot stock but no more. Once the stock is absorbed, add the same amount again; repeat this process, stirring regularly, until the rice is cooked through. Add extra hot water to the stock towards the end if it looks like you will need more.
Once the risotto is cooked, reduce the heat and stir through the pureed broad beans, the remaining half of the butter, the rest of the favas and the cheese. Cook over a low heat, stirring, until the butter and cheese melt and the puree is incorporated evenly. Season with salt.