Rub the pork with a little oil, then the rock salt and thyme. Roast for 30 minutes at 220C/fan 200C/gas 7 then turn the oven down to 160C/fan 140C/gas 3 and cook for 25 minutes per 500g. Rest for 20 minutes before carving.
Meanwhile, melt a knob of butter in a pan until foaming. Add the apple, a grating of nutmeg and a splash of water then cover and gently cook for 10-15 minutes until tender – mash with a potato masher to get the consistency you like.