1. Place half the oil in a frying pan over a medium heat. Add the onion and pancetta and cook, stirring, for 5 minutes, until soft and golden. Set aside to cool slightly, then transfer to a food processor. Add the Parmesan, breadcrumbs, parsley and egg. Pulse for a few seconds until combined and coarse. Season and cool completely.
2. Use your fingers to gently loosen the skin from the chicken breasts, then carefully push the stuffing under the skin. Pull the skin back over the stuffing, then cover and chill for about 30 minutes.
3. Preheat the oven to 180C.