Heat the oil in a deep frying pan and cook the onion and bacon for 5 mins to soften. Add the mushrooms and cook for a further 5 mins until they start to release their juices. Stir in the rice and cook until all the juices have been absorbed.
Add the stock, a ladleful at a time, stirring well and waiting for most of the stock to be absorbed before adding the next ladleful - it will take about 20 mins for all the stock to be added. Once the rice is cooked, season and serve with the grated Parmesan.