Bring the chicken stock to a simmer in a saucepan and keep it barely simmering over low heat.
In a casserole, melt 4 Tbsp butter. Cook the onions for 7 to 8 minutes. Do not let them brown. Add the rice and cook stirring until the grains glisten and are somewhat opaque.
Pour in the wine and boil it until it is almost completely absorbed.
Add 2 cups of the stock and cook uncovered, stirring occasionally, until almost all of the liquid is absorbed.
Add 2 more cups of the stock and cook, stirring occasionally.
Meanwhile, stir the saffron into 2 cups of stock and let it steep for a few minutes.
Pour over the rice and cook until the stock is completely absorbed.
By now the rice should be tender. If it is still firm, add the remaining stock-1/2 cup at a time-continue cooking and stirring until the rice is soft.
Stir in 4 Tbsp of soft butter and the grated cheese. Serve at once.