Preheat the oven to 475 degrees F (240 degrees C).

Pat the skin thoroughly dry. Score the skin with a sharp knife or ask your butcher to do it, cutting into the fat but not into the meat.

Separate the scored lines gently so salt can reach between them.

Put the pork in a baking pan skin side up.

Wedge the onion halves partly underneath it.

Rub the tablespoon of coarse salt over the skin and into the score marks.

Roast for about 25 minutes, watching that the skin doesn't scorch.

Now turn the heat down to 375 degrees F (190 degrees C).

Continue roasting until a thermometer in the thickest part of the loin, away from bone, reaches 145°F (63°C). A timing guide is about 35 minutes per pound including the first 25 minutes, but begin checking well before that estimate and remove the pork when it reaches temperature.

Use the thermometer reading to judge doneness; pinkness or clear juices alone aren't reliable.

Remove the pork from the oven and put it on a platter.

Rest for 30 minutes before carving, with any foil loosely over the meat rather than trapping steam against the crackling. The roast must rest for at least 3 minutes after reaching 145°F (63°C).

Spoon out and discard the fat from the baking dish, leaving the onion and meat juices in there.

If the roasting pan is stovetop-safe, set it over low heat and stir in the flour. Otherwise, transfer the skimmed juices and onion to a saucepan first. Never heat glass bakeware directly on the stove.

Stir it using a wooden spoon. Turn the heat up a bit and add the stock and cider.

Mix it using a balloon whisk.

Once it reaches a simmer, the gravy should be rich and smooth.

Grind in some salt and pepper to taste.

Discard the pieces of onion.

Serve the pork in slices with the gravy.