Heat chicken stock in a saucepan until simmering. Melt half of butter in a large heavy-based frying pan on medium. Cook mushrooms for 5 minutes, until golden. Remove from pan and set aside. Add onion to same pan and cook for 3 minutes. Stir through rice and cook for 1 minute, until rice is translucent.
Add 1 cup of hot stock. Simmer, stirring constantly, adding a ladle of stock at a time as liquid is absorbed. This should take 15-20 minutes, until rice is just tender.
Remove from heat and stir through grated parmesan, rocket, mushrooms and remaining butter. Cover and set aside for 2 minutes. Top with shaved parmesan to serve.