Heat the butter and oil together in a large saute pan over medium heat. Add the onion and saute until soft. Do not brown, they should be translucent. Add the rice and saute for a further 3 to 5 minutes making sure the rice is well coated, Using a ladle, add about 1/2 cup of stock to the rice and stir over a medium heat until the rice is almost dry and all the stock has been incorporated. Repeat this process until the rice is cooked but still has a slight bite. The rice should be creamy and moist, not runny, and the cooking time should be around 30 minutes.