Pour stock into a medium pan and place over a low, constant heat. Put saffron threads in a small bowl, add hot water and set aside.
Heat butter and oil in a large pan until butter is foaming. Add onion and stir with a wooden spoon for 2-3 minutes or until onion is soft. Add rice and stir for 1minute, then stir in saffron and liquid.
Add 1/3 cup hot stock, stir for 1-2 minutes, until rice has absorbed most of the stock continue adding 1/3 cup at a time, stirring frequently until rice is tender and creamy.