Melt 25g of the butter with the olive oil in a large heavy-based saucepan and add the onion. Cook over a gentle heat until the onion is soft but not coloured. Meanwhile bring the stock to the boil. Add the saffron to the hot stock and keep it on a simmer. Add the rice to the onion, stirring it for a couple of minutes until it looks glossy. Add the wine and stir until it has almost completely disappeared. Add the stock a ladleful at a time, stirring continually and only adding more once the previous lot has been absorbed. It will take about 20 minutes to stir in all the liquid and for the risotto to cook – it should be soft and creamy but there should be a very slight bite in the centre of each grain. Season to taste, then stir in the rest of the butter and a good handful of grated parmesan cheese, cover with a lid. Leave for five minutes then serve.