Preheat the oven to 190°C, fan 170°C, gas 5. Make a pocket in the pork for the stuffing: unroll the joint and place it skin side down. Using a sharp knife, cut a 7cm slit/pocket lengthways down the middle of the pork, leaving about 3cm on each side.
Mix the chopped onion, peppers, apple and parsley. Stir in 1 tablespoon of olive oil, stuff a third of the mixture into the slit and season. Roll up the joint so the stuffing pocket runs along the length and secure with string. Place the sliced onion in a large roasting dish and position the pork on top.
Roast the pork on the top shelf of the oven for 2 hours until the crackling is crispy and the pork is thoroughly cooked. Meanwhile, place the swede and potato wedges on another baking tray, drizzle with the remaining olive oil and season well. Place in the oven on the shelf below, once the pork has cooked for 1 hour.
Once cooked, transfer the pork to a serving dish to rest for 20 minutes, reserving the roasting dish and onions. To make a red pepper sauce, tip the remaining vegetable mixture on to a baking tray and cook for 15 minutes. Remove the mixture and whizz in a food processor until smooth, adding a little boiling water if the consistency is too thick.
To make a gravy, place the pork roasting dish with the onions over a medium heat, sprinkle with flour, stir and cook for 2 minutes until thickened. Add the wine, juice and 150ml of water and simmer for 8-10 minutes. Strain and serve with the roast pork, potatoes and swede, red pepper sauce and steamed greens.