Heat the water in a pot almost to boil; cover and keep it very hot on the stove, near the risotto pan.
Add olive oil, onion, and 1/2 teaspoon salt in the saucepan and set over medium heat.
Stir well as the onion starts to heat and soften; stir frequently and cook until it is wilted and just starting to color, 8 minutes or so.
Pour in the rice all at once, increase the heat, and stir the rice and onions continuously, toasting the grains (but not browning them), until they make a clicking sound as you turn them in the pan, 2 minutes or more.
Pour in the wine, and keep stirring for another couple of minutes, all around the pan, until the moisture has evaporated and the rice is dry.
Immediately ladle in 2 cups of the almost simmering water, enough to cover the rice, and decrease the heat.
Cook, stirring steadily, until the water is almost totally absorbed, 4-6 minutes.
Quickly ladle in more water to cover the rice, add another 1/2 teaspoon salt, and keep stirring, as the rice swells and releases its starches and a thick creamy suspension starts to form.
Again, when the water is almost completely absorbed--and you can see the bottom of the saucepan as you stir--ladle in another cup or so of water.
Remember how much water you add; after incorporating 6 cups (or a bit more) over a period of 15-19 minutes, taste the rice for texture and seasoning--add more salt and/or incorporate more hot water if needed.
When the risotto is perfectly cooked--at once al dente and creamy--turn off the heat.
Without delay, drop the butter pieces into the saucepan and stir vigorously to mount--or amalgamate--the risotto with butter.
Stir in the 1/2 cup cheese.
For each serving, spoon a mound of risotto into a warm pasta bowl, and immediately shave paper-thin flakes of truffle over the top.
Serve right away, as the heat releases the aroma of truffle into the air, and dish up the next portion.