1. Several hours or the night before you plan to serve the croquettes, make the risotto:  Melt the butter in a heavy saucepan over medium heat.  Add the onion and cook, stirring, until it has softened, 5 to 10 minutes.  Then stir in the rice, the stock and the salt (if you are using canned broth, taste it for saltiness and adjust accordingly).  Raise the heat and bring to a boil.  Immediately reduce the heat, cover, and simmer until the liquid is absorbed and the rice is tender, 10 to 20 minutes.  Lift the lid and stir the rice occasionally.  If the rice is not tender but the liquid has been absorbed, add more liquid and cook, stirring constantly, until it softens.

NOTE:  If you make the risotto the night before, you can chill it in the refrigerator and it will be much easier to form into balls.