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Heat oven to 160C/140C fan/gas 3. Sit the pork on a sheet of foil in a roasting tin. Mix the butter, sugar and mustard with 2 tsp salt, rub all over the top and ends of the joint (or joints), then scrunch up the foil to seal tightly in a parcel.
Mix the onions and apples in a large, shallow roasting tin. Scatter the peppers over the top and poke in some bay leaves. Mix the vinegar, oil, mustard and sugar with 100ml water and drizzle over everything. Roast the pork for 3½ hrs while you cook the peppers on the shelf below for the first 2 hrs. When you remove the apples and peppers after 2 hrs, set aside and put the Salt-baked potatoes (see 'Goes well with', right) into the oven underneath the pork instead.
After 3 hrs, unwrap the foil from the pork. Scrape any mustard mixture stuck to the foil back onto the pork, sit it and any juices back in the tin and increase oven to 200C/180C fan/gas 6. Roast for 30-45 mins more until the skin is brown and crisping. Remove the pork from the oven, cover and rest for 20 mins while you put the peppers back on the top shelf for 20 mins to finish. Serve the pork thickly sliced alongside the apples and peppers, with any tin juices poured over.