Step 1
To make the pickled onion, in a medium saucepan, bring the vinegar, sugar and 2 tsp sea salt flakes to a boil, whisking until sugar dissolves. Place onion in a medium bowl and pour over the hot pickling liquid. Stir to combine. Set aside to cool to room temperature, tossing occasionally. Place in the fridge to chill.
Step 2
Meanwhile, position a rack in the centre of the oven. Preheat oven to 150°C (130°C fan-forced). Place pork in a flameproof roasting pan. Season well with salt. Roast pork for 1 hour 10 mins or until an instant-read thermometer inserted into the centre of the pork registers 45°C.
Step 3
Remove pork from oven. Increase oven temperature to 250°C (230°C fan-forced). Return pork to oven. Roast for 35-40 mins or until skin is crisp and crackling and an instant-read thermometer inserted into the centre of the pork registers 60°C and juices run clear. Transfer to a carving board and set aside for 15 mins to rest.
Step 4
While the pork is resting, heat the roasting pan over medium heat on a stovetop burner. Add the apple juice. Stir, scraping any browned bits on the base of the pan. Carefully strain into a jug and stand for 5 mins. Skim off any fat that rises to the surface.
Step 5
Transfer apple juice mixture to a saucepan over medium heat. Bring to a simmer. Cook for 5 mins or until liquid has reduced to ¾ cup (185ml). Stir in butter and season with salt and pepper. Keep sauce warm.
Step 6
Very thinly slice the fennel, apple, and celery (use a mandolin or vegetable slicer if you have one). Transfer into a large bowl. Drain the pickling liquid from the onion mixture. Toss the onion, parsley, lemon zest, 2 tbs lemon juice and oil with fennel mixture in the bowl to combine. Season with salt and pepper.
Step 7
Arrange salad over a serving platter. Using a serrated knife, slice pork into 2cm-thick slices and arrange over the salad. Serve with the sauce.
Make it ahead
Make the pickled onion for the salad up to 3 days ahead and store in the fridge. Drain and toss with the salad ingredients just before serving. Pickled onion is also delicious on sandwiches with leftover roast pork.