Preheat the oven to 200°C, fan 180°C, gas 6. Put the onions in a deep roasting tin. Remove the rind from the pork, leaving a thin layer of fat.
Mix the rosemary with the fennel, stock cube and black pepper, and rub into the pork.
Sit the pork, fat side up, on the onions. Pour 1.5 litres water into the tin, so it comes halfway up the pork. Roast for 20 minutes, then take out of the oven and tightly cover the tin with foil. Reduce the oven temperature to 150°C, fan 130°C, gas 2 and cook the pork for a further 4 hours.
Turn the oven up to 200°C, fan 180°C, gas 6. Transfer the pork to a baking tray (reserve 500ml of the pan juices and the onions). Mix 1 tbsp maple syrup with the mustard and brush over the pork. Return to the oven for 30 minutes.
Meanwhile, simmer the apples in a large pan with the remaining maple syrup and 100ml water for 20 minutes.
Shred the pork using 2 forks, then pour over the reserved pan juices and onions. Serve in rolls with the toffee apple sauce.