In a pan, sauté the finely chopped onion with two tablespoons of oil, then add some stock to keep it soft. Add the rice and toast it, then pour in the white wine and allow it to evaporate.
Start pouring the stock into the pan, one ladleful at a time, waiting each time until the previously added stock has been almost completely absorbed. Stir constantly.
When cooked (about 20 minutes), turn off the heat and add the butter and grated Parmesan cheese.
Leave the risotto to rest for about 5 minutes in the covered pan.
Serve the risotto with a few drops of Extra Aged Traditional Balsamic Vinegar of Modena P.D.O.