Score pork skin widthways at 1cm intervals with a small sharp knife, rub in 1 tbsp sea salt flakes and refrigerate, uncovered, to dry out (overnight).
For rhubarb and rosemary jelly, bring rhubarb, apple, lemon juice and seeds and 1.5 litres water to the simmer in a large saucepan over medium-high heat. Reduce heat to medium, simmer until pulpy (1-1¼ hours), strain through a muslin-lined sieve
to yield 1 litre liquid (do not press on solids; discard solids). Transfer rhubarb liquid to a clean pan, add sugar and stir over medium-high heat until sugar dissolves. Simmer until mixture reaches setting point (45 minutes-1 hour); to test, spoon a little on a chilled saucer, return saucer to freezer for 1 minute, then push edge with your finger. If mixture wrinkles, it is ready. Remove from heat, stand for 10 minutes, skim scum from surface and stir in vinegar. Ladle into sterilised jars, add a rosemary sprig to each, seal and stand until cooled to room temperature. Makes about 500ml. Jelly will keep for 3 months; refrigerate after opening.
Preheat oven to 200C. Cut two onions into thin wedges, combine with potatoes and half the oil in a roasting pan, season to taste and roast while you prepare the pork.
Finely chop remaining onion, add with garlic and remaining oil to a small saucepan and stir occasionally over medium heat until tender (3-4 minutes). Transfer to a bowl, add breadcrumbs, rhubarb and parsley, season to taste and stir to combine.
Lay pork skin-side down on a work surface, scatter breadcrumb mixture along centre, then roll to form a long cylinder. Secure at intervals with kitchen string, place on top of potatoes and onion in roasting pan and roast until skin starts to crisp (25-30 minutes). Reduce oven to 180C and roast until just cooked through (50-55 minutes). Serve hot with rhubarb and rosemary jelly.