Pour ½ cup of the hot stock over the saffron in a small heatproof bowl. Let it stand. In a heavy, wide 3- to 4-quart casserole or pot, heat the olive oil over medium heat. Add the onions and shallots and cook them until golden, stirring often, about 8 minutes. Add the rice and stir to coat with oil. Toast the rice until the edges become translucent, 1 to 2 minutes. Pour in the wine and stir well until evaporated. Add ½ cup of the hot stock and ½ teaspoon of the salt. Cook, stirring constantly, until all the stock has been absorbed. Stir in the beef marrow, if using. Continue to add hot stock in small batches— just enough to completely moisten the rice— and cook until each successive batch has been absorbed. About 10 minutes after adding the first addition of stock, stir in the saffron mixture. Stir constantly and adjust the level of heat so the rice is simmering very gently while adding the stock until the rice mixture is creamy but al dente. This will take 16 to 20 minutes from the time the wine was added. Remove the casserole from the heat. Beat in the butter until completely melted and then the cheese. Adjust the seasoning with salt, if necessary, and pepper. Serve immediately, ladled into warm shallow bowls. NOTE: Few butchers sell beef marrow separately any more, but it is easy to come up with your own. To extract about 2 ounces of marrow, start with five to six 1- to 1 ½- inch beef marrow bones. (Your butcher can supply them and cut them to the correct size.) Heat the beef stock for the risotto and warm the bones in the stock two at a time for 3 minutes. Remove the bones with a slotted spoon and let them stand just until cool enough to handle. Try to push the marrow through the bone with your finger; it should come right out. If not, use a spoon to scoop it out. The marrow may be soft around the edges, but should the center should still be firm enough to chop.