Heat butter and oil in a deep heavy saucepan and cook onion until soft.
Add rice and cook for a further minute, stirring to coat rice well.
Stir in 2 cups of stock and add saffron infusion (as per packet instructions) ensuring to stir and mix well.
Add in remainder of stock. Stir well, cover tightly and cook gently for 20 minutes. Remove lid and stand for a few minutes.
Fluff rice with a fork and serve.