In a small bowl mix together oil, sage, garlic and tomato paste. With a knife, make small incisions into the pork. Rub in mixture and season well.
Lay prosciutto on the chopping board. Place pork on top, and fold over so the pieces of prosciutto wrap around the pork. Tie and secure with kitchen string.
Heat a medium sized heavy based baking dish onto the heat. Add pork and cook, turning, until browned. Add white wine and let sizzle. Add stock and cover tightly with foil.
Bake for 3 1/2 to 4 hours (210-240 minutes) or until tender, checking that liquid isn't evaporating too quickly.