Put the stock and saffron in a saucepan over a medium-high heat and bring to just below the boil, then reduce the heat to a simmer.
Meanwhile, heat the oil and half of the butter in a frying pan over a medium heat. Add the onion and fry, stirring, for 3–5 minutes until softened. Add the rice and stir until coated. Add the wine and leave it to bubble until it evaporates.