Heat 30 g of butter in a heavy-bottomed 3-quart saucepan over low heat. Add the diced onion and cook, stirring occasionally, for 8 to 10 minutes until completely soft and translucent but not colored.

Add the Carnaroli rice to the pan and raise the heat to medium. Stir constantly for 60 to 90 seconds until the grains look slightly translucent at the edges and smell faintly nutty.

Pour in the white wine all at once. Stir until the liquid is fully absorbed and the sharp alcohol smell has cooked off, about 2 minutes.

Add one ladle (about 80 ml) of warm stock to the rice. Stir steadily until nearly all the liquid is absorbed before adding the next ladle. Continue this process, ladle by ladle, over medium heat.

After about 16 to 18 minutes total cooking time, taste a grain of rice. It should be cooked through with just a slight firmness at the very center - al dente. The consistency of the risotto should look slightly soupy at this stage; it will tighten during mantecatura.

Add one final small ladle of stock if needed to keep the mixture loose, then remove the pan from the heat.

Add the cold butter and grated Castelmagno to the pot off the heat. Stir vigorously in a circular motion for 90 seconds until the cheese and butter have fully melted into the rice and the surface looks glossy and cohesive.

Taste and adjust salt. Cover the pot and rest for 2 minutes. The risotto should flow slowly when you tilt the pan - not sit in a mound and not run like soup.

Spoon into warm shallow bowls. Finish with a little extra grated Castelmagno and a few turns of black pepper if you like. Serve immediately.