In a large skillet or saucepan, melt the butter or heat the olive oil over medium heat.
Add the diced onion and sweat them, stirring occasionally, until they are translucent and fragrant: about 5-7 minutes. Be careful not to brown the onions.
Add the arborio rice to the skillet and stir to coat it with the fat. Cook for about 2 minutes, stirring frequently, until the rice becomes translucent around the edges.
Begin adding the hot chicken stock to the skillet, one ladleful at a time, stirring constantly and allowing each addition to be absorbed before adding more. This process will take about 20 - 25 minutes. The rice should be creamy and tender with a slight bite to it. (Al dente).
Once the rice is cooked to your desired consistency, stir in grated parmesan cheese to taste and season with salt and pepper.
Serve the risotto immediately, garnished with additional grated parmesan cheese if desired.