Heat oven to 220C/200C fan/gas 7. Take the pork out of the fridge and pat the skin dry. Put the rosemary leaves, salt and pepper in a mini chopper and grind together (or do this with a mortar and pestle). Rub the rosemary salt all over the pork, making sure it gets into the cuts in the skin. Sit the pork in a large roasting tin, ideally on a rack, and roast for 30 mins.
Reduce heat to 170C/150 fan/gas 3 and roast for 1½ hrs more. Turn oven back up to 220C/200C fan/gas 7 and roast for 20-30 mins to crisp. Leave to rest on a board for 10 mins before carving.
When the pork is nearly ready to come out, put the celeriac and potato in a large pan of water, bring to the boil and cook for 10 mins until just tender. Add the pears and cook for 2 mins more. Drain well, then mash until smooth – a mouli or potato ricer does this job best. Beat in a splash of cream and some butter, and serve with the pork and broccoli.