Preheat the oven to 200°C.
Mix walnuts, apples, thyme, and two-thirds of the butter into a chunky paste and season.
Butterfly the pork loin and stuff with the butter mixture.
Wrap the pork in greased foil, rub with remaining butter, and tie tightly with string.
Sear the foil parcel in a hot roasting tin with vegetable oil until browned.
Roast the pork in the oven for 60 to 90 minutes until cooked through.
Coat parsnips in olive oil and honey, then season with salt, pepper, and thyme.
Roast the parsnips for 35 to 40 minutes until golden.
Add walnut halves to the parsnips and roast for another 5 minutes.
Slice the pork and serve on plates with the parsnips and gravy.