Put all the spice ingredients into a food processor and blend together to make a smooth, wet paste.
Slash the rind of the pork as much as you can with a very sharp knife (to make good crackling).
Rub the spice mix all over the meat, but not the rind, and marinade for at least 1 hour at room temperature or overnight in the fridge.
When read to roast, dry the rind of the pork and rub generously with salt.
Heat oven to 190ºC and roast in the oven, according to the packet instructions (x2).
Meanwhile, toss the sweet potato and onion together in the oil. Scatter around the pork and return to the oven for about 40 minutes, until the potatoes are tender.