Preheat oven to 220ºC. Place pork rack on a roasting pan, skin side up, season well with sea salt and, using your fingertips, rub salt into slashed fat to get the perfect crackling.
Next, rub in ground black pepper and fennel seeds, then rub all over with oil.
Roast for 30 minutes. Remove roasting pan from oven, move pork to one end and tuck garlic and sage underneath.
Sit apples at the other end of pan and cover with foil, leaving meat uncovered. Turn heat down to 190ºC and continue to roast for a further 45 minutes.
Remove pork from pan and allow to rest, loosely covered with foil, for about 10 minutes.
Tilt roasting pan and spoon out fat. Add half the cider to pan and simmer over a medium heat for 5 minutes, scraping pan with a spoon and turning apples occasionally, until liquid is reduced and sticky, and apples are lightly caramelised. Add remaining cider and simmer for about 3 minutes until gravy reduces a little.
Carve pork rack into 4 chops and serve with apples, garlic and sage gravy, and mashed potato and steamed greens if desired.