Put the stock in a pan and keep at simmering point. Heat the butter in a large heavy-based pan. Cook the onion for 2–3 minutes, until softened but not browned, then add the rice and stir for 2–3 minutes to coat.
Add the wine and saffron to the rice and stir until absorbed. Stir in a ladleful of stock and stir until absorbed, then add another la