Bring the stock to a boil, then reduce the heat and simmer. In a shallow saucepan, sauté the onion and marrow in 3tbsp/45ml of the butter until the onion is soft. Add the rice and sauté lightly for 1 minute, stirring with awooden spoon.
Over medium heat, slowly add the wine and one third of the simmering stock. Lower the heat and simmer uncovered until the liquid is absorbed, about 5 minutes. Add another third of the stock and simmer, still stirring, 10-12 minutes. Meanwhile, add the saffron to the remaining stock and leave to infuse. When the rice has absorbed all the liquid, add the saffron-flavored stock and salt and pepper to taste. Continue simmering and stirring until the rice thickens, about 20 minutes. The grains of rice should be tender but firm and the mixture creamy but just stiff enough to hold a shape. If the rice thickens before it is cooked, add more hot stock and continue simmering.
Remove from the heat. With a fork, stir in the remaining butter and the Parmesan. Serve at once.