Cook the pancetta on a baking sheet in a preheated oven, 200°C (400°F), Gas Mark 6, for 5–6 minutes, or until golden brown. Set aside.
Melt half the butter in a heavy-based saucepan over a low heat. Add the onion and cook for 10 minutes until softened. Add the garlic and rice and cook, stirring, for 1 minute. Pour in half the wine and cook, stirring, until absorbed. Add the remaining wine and cook, stirring, until absorbed.
Stir in the radicchio and 3 ladlefuls of the simmering stock. Slowly simmer, stirring constantly, until the stock has been absorbed and the rice parts when a wooden spoon is run through it. Add another ladleful of stock and continue to cook, stirring and adding the stock in ladlefuls, for 18–20 minutes until the rice is creamy and almost tender to the bite.
Remove from the heat and add the remaining butter and the Parmesan. Stir vigorously for 15 seconds. Cover with a tight-fitting lid and leave to stand for 1 minute. Serve immediately topped with the pancetta, with extra Parmesan on the side.