Preheat oven to 225°C. Heat oil in a non-stick frying pan over a medium heat. Add leek and cook for 4-5 minutes on a medium-low heat until soft but not brown. Transfer to a bowl. Cook speck in pan on a medium heat for 3-4 minutes or until golden. Add speck, walnuts, 2 teaspoons of seasoning and breadcrumbs to bowl with leek. Rub in butter until combined.
Make a 6cm-deep cut between bone and flesh of each rack. Fill cavities evenly with stuffing. Pack down firmly. Hold flesh and bone together and tie up firmly between each cutlet with kitchen string to secure. Rub the exposed flesh with remaining seasoning.
Put on a rack in a roasting pan. Grease pork rind with oil and rub in salt. Roast for 30 minutes until crackling forms. Reduce heat to 180°C and roast for a further 50-55 minutes or until cooked through. Meanwhile, put apples on an oven tray and spray with olive oil. Add to oven in the last 15-20 minutes of pork cooking time, and roast until soft.
Allow pork to rest for 10 minutes before carving. Cut apples in half and serve with pork, with pan juices spooned over.