Start by making the Royal pasta dough. While it rests, make the filling.
Trim and quarter the radicchio, peel the onion and slice into eighths, then place in a small roasting tray. Pour over the balsamic and two tablespoons of olive oil, toss to coat, then strip over half the thyme leaves, reserving any pretty tips.
Cover with a sheet of wet scrunched-up greaseproof paper and roast for one hour.
Prick the potatoes and dry-roast in a separate tray alongside until cooked through, then remove. Once cool enough to handle, scoop the potatoes out of their skins and mash the insides in a bowl.
Finely chop the radicchio and onion, then stir into the potato with one tablespoon of the balsamic juices from the tray. Finely grate in most of the Parmesan and all the fontina, mix well, then season to perfection with sea salt and black pepper. Leave to cool completely.
On a clean flour-dusted surface, roll out the pasta dough so it’s one millimeter thick and roughly 16 centimeters wide (the width of a standard pasta machine).
Spoon heaped teaspoons of filling evenly down the pasta strips, slightly off centre, leaving a five centimeter gap between each. Brush the exposed pasta lightly with water and fold the sheets in half over the filling.
Gently seal around the filling, pushing out the air – you can cut them into eight centimeter circles with a pastry cutter or into squares with a knife. Place them on a semolina-dusted tray as you go – you’ll get about 18 ravioli in total.
Toast the hazelnuts in a large frying pan on a medium heat until golden, then lightly bash and tip into a bowl.
Cook the ravioli two portions at a time in a pan of boiling salted water for three to four minutes.
Meanwhile, place the frying pan back on a low heat and add a splash of olive oil, a third of the butter, and a third of the remaining thyme leaves and tips. Fry until golden, then remove from the heat.
Scoop the pasta straight into the buttery sauce, bringing some starchy cooking water with it, add a fine grating of Parmesan, toss gently, then spoon on to warmed plates.
Finish each portion with a scattering of hazelnuts, a little extra virgin olive oil, some extra Parmesan and a few small drips of the balsamic juices, if you like. Serve up to your first lucky guests, while you crack on with the next two portions.