Separate the radicchio leaves, wash them, dry them and remove the hard white core part of the leaves. You should get 150 g (5 1/2 ounces) of leaves (only the red part as shown in the photograph).
Chop the leaves, possibly making long and thin strips.
Cut the cheese into very thin slices.
Put 50 g (2 ounces) of the butter in a large pan and melt it (keep the remaining butter for later).
Add the finely chopped onion.
Sweat the onion over a medium heat for about 3-5 minutes or until it is soft.
Then, add the radicchio.
Wilt the radicchio for 2-3 minutes; you will see that it will reduce its volume by about 1/3.
Then, add the rice...
...and stir until the rice is completely coated with butter. You can see that the rice will start to become translucent. Keep stirring and sweat the rice for a minute or two.
Then, add the white wine, keep stirring and...
...let the wine evaporate (it will probably take a couple of minutes).
Now, add the stock and turn the heat to medium/low. The stock, which is kept simmering in order to stay hot, should be added at the rate of a couple of ladlefuls at a time and when this is absorbed, add more stock. Carry on in this way for about 15 minutes. During this stage, do not leave the pan alone and stir regularly (gently).
After 15 minutes, you will probably need an extra 3 to 5 minutes to complete the final stage of cooking (this time depends on the type of rice you are using). From now on, taste the rice every minute until the rice is cooked “al dente” (this means that the rice is tender outside, but still slightly firm to the bite in the centre). At the same time, the tasting will tell you if you need to adjust seasoning with salt. Regular but gentle stirring is required for the last minutes in order to avoid the risotto sticking to the bottom of the pan.In these last minutes, if the stock is completely absorbed, you can gradually add a bit more (say a half ladle at a time) because at this final stage you do not want the risotto becoming too watery.
At about 1 minute before you put the cooker off, add the slices of cheese.
Stir for few seconds until the cheese is completely melted.
When the rice is perfectly cooked "al dente", put the cooker off and add all the remaining butter (cut into pieces) and stir until completely melted.
Cover the pan with a lid for about 1 minute and allow the risotto to rest before serving it.
Risotto should be served "all'onda" (like a wave). This means that when you serve it onto the plate, it should still flow a little.