Preheat the oven to 220ºC, fan 200ºC, gas 7. Put the pork in a large roasting tin and pat the rind dry with kitchen paper. Rub the sea salt into the scored skin and roast in the oven for 30 minutes. Reduce the oven temperature to 190ºC, fan 170ºC, gas 5 and cook the pork for a further 1 hour.
Meanwhile, put the potatoes in a large pan of cold water, cover, and bring to the boil. Cook for 8-10 minutes or until soft on the outside but still hard in the centre. Drain and return the potatoes to the pan. Add the sesame seeds to the pan and shake so that the potatoes rough up a bit.
Remove the pork from the oven, add the sesame-coated potatoes, unpeeled garlic cloves and onions to the tin, then drizzle the veg with the oil.
Return to the oven and cook for 45 minutes until cooked through.
Remove and let rest for 20 minutes before carving. Carve 110g pork per person and serve with the crackling, potatoes, onion and garlic.