Heat the oil over medium heat. Add the onion and cook until soft. Heat the stock in another pot and add the saffron. Add the rice to the onions and stir until toasted and translucent. Add the wine to the rice and simmer. Add enough stock to just cover the rice and cook, stiring until it is absorbed. Continue doing this until the rice is al dente. Stir in the butter, cheese, and any vegetables you want to add.