Bring the chicken stock to a simmer in a 2- to 3-quart saucepan and keep it barelysimmering over low heat.
In a heavy 3-quart flameproof casserole, melt four tablespoons of butter over moderateheat.
Cook the onions in the butter, stirring frequently, for 7 to 8 minutes. Do not let thembrown.
*Stir in the optional marrow, if you are using it. (I do not bother with this.)
Then add the rice and cook, stirring , for 1 or 2 minutes, or until the grains glistenwith butter and are somewhat opaque. Pour in the wine and boil it until it is almostcompletely absorbed. (If you prefer the rice without wine, just substitute one-half cup ofstock for the wine. Then add 2 cups of the simmering stock to the rice and cook uncovered,stirring occasionally, until almost all of the liquid is absorbed.
Add 2 more cups of stock and cook, stirring occasionally.
Meanwhile, stir the saffron into 2 cups of stock and let it steep for a few minutes. Thenpour it over the rice.
Cook until the stock is completely absorbed.
By now the rice should be tender. If it is still firm, add the remainingstockone-half cup at a timecontinue cooking and stirring until the rice issoft. Stir in remaining four tablespoons softened butter and the one-half cup gratedcheese with a fork, taking care not to mash the rice.
Serve at once while the rice is creamy and piping hot.