The seal is everything -- wrap tightly in parchment then foil with no gaps. Low and slow is non-negotiable: 160C/325F fan for 3.5 to 4 hours. Do not open the oven during cooking. Shoulder is preferred over leg for its intramuscular fat and collagen. Overnight marinating improves depth of flavor. Open the parcels at the table, not in the kitchen. The cooking juices are already the sauce -- nothing separate is needed.