Have all the ingredients at room temperature or warmed before you start cooking. If the ingredients are cold, it shocks the rice and causes it to stay hard at the core. It also slows down the process and inhibits the release of the essential starch.
The finished risotto is sensitive to timing. It should be served immediately, not partially cooked and then finished just before serving
1. Sauté the onion: In a wide, heavy saucepan over medium heat, heat the oil and/or butter. Add the onion and sauté until softened. (Sometimes green onions or leeks are used in place of the onion.) Be sure the onion does not overbrown; just cook it enough to soften and introduce its flavor subtly to the dish.
2. Coat the rice: Add the rice to the pan with the onion. Stir until the grains are well coated with oil and/or butter and are translucent with a white dot in the center, about 3 minutes. Lightly toasting the rice in fat tempers the protein coating and coats the grains with the fat, a liquid-resistant substance that will inhibit overly rapid absorption of the cooking liquid. This allows even release of the starch for a more consistent creaminess in the risotto. Never allow the rice to brown.
A splash of wine is often added at this point to deglaze the pan, loosening any cooked bits stuck to the bottom. Stir until the wine is completely absorbed.
3. Add the stock: Add gently simmering stock to the rice, a ladleful at a time, stirring frequently after each addition. Adjust the heat so the rice cooks briskly but not so quickly that the grains start to fall apart. Keep the grains bagnato, bathed in broth, so they don't dry out, but wait until the stock is nearly fully absorbed (but the rice is never dry on top) before adding the next ladleful. The slow addition of the liquid controls the release of starch; what isn't absorbed evaporates. Keep stirring frequently, which helps the fat and starch join and keeps the rice from scorching.
Rice continues to cook even after being placed in a serving dish and continues to release starch and absorb liquid, so reserve about 1/4 cup stock to add at the end.
4. Stir in the final ingredients: The rice is done when it is tender to the bite but slightly firm in the center and looks creamy; this takes about 20 minutes. At this point, add any reserved vegetables or other ingredients called for. Cook to heat through, remove from the heat and add a tablespoon of butter, freshly grated Parmigiano-Reggiano cheese (if using) and the reserved 1/4 cup stock. Season with salt and pepper and serve immediately.