Heat first butter and olive oil over medium heat until it melts; cook onions until translucent. Add the rice and cook until lightly toasted and fat has been absorbed, 1 to 2 minutes. Add the white wine, stir until fully absorbed. Add the chicken stock, one fourth at a time, stirring often. Allow stock to be completely absorbed before adding more stock. Halfway through cooking (after 6-8 minutes) dissolve saffron in a little hot broth, let soak 3 minutes and add to the rice. Do not add at the beginning. Stir the rice frequently, so it does not stick to the bottom of the pan. From start to finish, this dish should take 18-20 minutes. It may not take all of the chicken stock, it may take more. When the rice is cooked al dente, turn off heat, vigorously beat in parmesan, add butter and remove from heat. Correct seasoning and let stand, covered, for a few minutes so the rice finished cooking.