Roughly chop the Fontina, whisk the egg yolks with a couple of tablespoons of milk, blend the flour into the remaining milk and add the fontina to the mixture. Transfer the mixture over the heat and keep whisking vigorously with a metal whisk.
As soon as the mixture starts to become more solid, amalgamate everything with the egg mixture, and, cook for ten minutes, stirring continuously. Season with salt, if you want you can add a knob of butter, and leave to rest.
Finely chop the radicchio and add it to the cheese mixture. Heat up the fondue and fill the vol au vents finishing with some freshly-grated black truffle on top.