1. Gently soften the onion and garlic in olive oil, using a non-stick frying pan. Add the olives, tomatoes and rosemary. Continue to cook for another 2-3 minutes. Cool. Pre-heat the oven to 175 degrees C
2. On a chopping board, remove all the string from the pork joint and unroll, top side down. Spoon the onion filling in a line down the middle, re-roll tightly and tie. Don't worry if some of the filling oozes out, just pack it back in agan after tying.
3. Place the pork in a roasting tin, right side up and remove the visible fat. With your fingers, gently pat the ground walnuts in place, with a final press to ensure they stay in place.
4. Sprinkle with olive oil and roast in the pre-heated oven for about 1 - 1 1/4 hours or until the juices run clear.
5. Remove from the oven, cover with a cloth and allow to rest for 10-15 minutes before serving.
Rolling and Stringing the pork: Detailed insructions can be found in the recipe for Herbed Stuffed Pork