Cook fusilli in a large saucepan of boiling, salted water according to packet directions. Drain and return to pan, covering with a lid.
Meanwhile, heat oil in a small frying pan and sauté pancetta for 4 minutes, until golden. Set aside, tear into pieces. Add onion and cook another 5 minutes, stirring. Add garlic and cook another minute. Increase heat to high, add radicchio with ½ cup water and cook for another 4 minutes, until wilted. Add balsamic vinegar, season to taste and remove from heat.
Add radicchio to pasta and toss on high heat for 1 minute. Serve immediately.