Put the pork on a rimmed baking sheet and sprinkle all over with 1 tablespoon salt and several grinds of pepper; set aside. Put the fennel, allspice, whole peppercorns and juniper berries in a small skillet over medium heat and cook until the fennel begins to turn slightly golden, about 5 minutes. Transfer the toasted spices to a blender, add the bay leaf and blend until coarsely ground. Add the olive oil and 1 tablespoon salt and process until a coarse paste forms, about 1 minute. Spread the paste over the pork, making sure to get it into all the nooks and crannies; use a spatula to scrape up all the paste from the blender. Cover the pork with plastic wrap and refrigerate at least 8 hours and up to 24 hours. When ready to cook the pork, preheat the oven to 450 degrees F. Put the pork on a rack set in a large roasting pan and cook for 20 minutes. Lower the oven temperature to 350 degrees F and continue to cook until an instant-read thermometer registers 135 degrees F, about another 1 1/2 hours. Meanwhile, combine the sugar, sage, thyme, apples, onions and 6 tablespoons of the butter in a large bowl until well coated. Heat a large skillet over high heat until very hot, about 3 minutes. Add half of the apple and onion mixture and cook, stirring occasionally, until the apples and onions are dark