Preheat the oven to 200°C.
Butterfly the pork loin by cutting vertically through the thick part to open it like a book.
Spread minced garlic and fresh oregano leaves over the surface of the meat.
Lay Parma ham slices horizontally over the pork.
Sprinkle chilli flakes over the ham and roll the loin tightly.
Secure the roll with string tied at 4cm intervals.
Place sliced unpeeled onions and oregano stalks in a roasting tin to form a base.
Set the pork on the onions, drizzle with oil, and roast for 75 minutes until the internal temperature reaches 71°C.
Deglaze the tin with vermouth and boiling water to create a gravy.
Rest the meat in the sauce for 15 minutes before carving into 2cm slices.