Keep the broth at a slow, steady simmer on a burner near the risotto pot.
In a broad, sturdy pot over medium-high heat, cook the onion with the oil and 1 tbsp of the butter until translucent. Add the rice and stir quickly to coat the grains well.
Add 1/2 cup (120 ml) simmering broth and stir constantly, wiping the bottom and sides of the pot as you go, until the liquid is gone.
Add another 1/2 cup (120 ml) broth and keep stirring in the same way, maintaining a lively pace. Repeat, adding broth as each addition is absorbed and evaporates.
Begin tasting the rice after 20 minutes of cooking. Keep cooking until the rice is tender but firm to the bite. As it nears doneness, reduce the amount of broth added so the finished risotto is slightly moist, not runny.
When the rice is 1 to 2 minutes from done, add all the Parmesan and the remaining butter. Stir constantly until the cheese melts and clings to the grains. Off heat, correct salt if needed.
Turn onto a platter and serve promptly. If using truffle, shave it over the risotto (or over each portion).
