Heat half the butter and the oil in a large, heave-based saucepan over-low medium heat. Add the onion and saute until softened. Add the peas and cook for 10 minutes. Place half of cooked peas in a food processor and blend until pureed. Set aside.
Add the rice with the remaining peas, stirring to coat, and cook for a further 2 minutes. Add the stock gradually, a ladle at a time, stirring constantly. Be sure the liquid is absorbed before the next addition. Cook for 10 minutes, adding the stock.gradually, stirring continously. Add the puree and cook for another 5-10 minutes, adding any remaining stock, until rice is al dente.
Mix half the parmesan, the remaining butter, the parsley and mint. Season with salt and pepper. Served topped with the remaining cheese.