If you take one thing from this recipe, I hope it's the method of curing the meat with salt and sugar. Yes, it involves planning ahead, but the work and mystery proceed unattended in your refrigerator, the meat achieving fantastic savor and juiciness.
The pork must be cured overnight, so start this recipe the day before.
The day before you plan to make the pork, combine the salt, sugar, and sage in a small bowl, and rub it well all over the pork. Wrap the pork in plastic wrap, or otherwise seal it up airtight, and refrigerate it overnight. Remove it from the fridge about an hour before you plan to cook it, if you think to, so it starts off at room temperature. (If you forget, it doesn't really matter).
Heat the oven to 400 and spray either an oven-proof skillet or a stove-proof roasting pan with olive oil. Place the pork in the pan, surround it with the apples and onions, give everything a final misting of olive oil, and pop it in the oven.
After half an hour, flip the roast over and stir up the apples and onions, then roast for another half an hour. Now remove the pork from the pan to a cutting board, tent it with foil so it stays warm, and make the sauce.
Over medium heat, add the cider to the pan full of dripping, apples, and onions, and boil, scraping the pan, until the cider is reduced by half and the pan is full of something that seems kind of like a thinnish, darkish applesauce. Add the cream and simmer very gently, whisking to combine everything, then taste for salt (you will likely need to add some) and pour it into a bowl with a spoon for serving.
Carve the pork into think slices and serve.