Cook the fava beans in a saucepan of lightly salted boiling water for 2-3 minutes. Drain and plunge into ice-cold water to cool. Peel away and discard the outer shells and set the beans aside.
Heat the butter in a heavy saucepan. Add the onion and bacon and cook over low heat for 5 minutes. Add the rice and beans and continue to cook, stirring for 1-2 minutes.
Add enough hot stock to cover the rice and continue to cook, stirring frequently, until most of the stock has been absorbed. Continue adding the stock in this way until it is almost completely absorbed and rice is tender.
Remove from the heat, stir in the grated Parmesan, and season to taste. Serve immediately, garnished with Parmesan shavings.