Place chopped onion in a deep saucepan with butter and diced beef marrow. Cook slowly for 2 to 4 minutes, taking care that the onion does not become brown. Add the rice and cook over medium heat, stirring constantly with a wooden spoon. After a minute or so stir in a cup of hot beef stock in which you have dissolved the powdered saffron.
Continue cooking, adding stock as needed and stirring from time to time, until rice is cooked (15 to 18 minutes). Correct seasoning. By this time all the stock in the pan should have been absorbed by the rice, leaving rice tender but still moist. Serve rice immediately with extra butter and freshly-grated Parmesan.