IF SKIN IS STILL ON: Place the pork on a clean work surface, skin-side up. Using a small very sharp knife, make scores about an inch apart through the skin into the fat, but not so deep that you cut into the meat. If your cut doesn’t have skin, skip this step. You can score in strips or cross-hatch, depending on how you want your crackling.
Crush fennel seeds and peppercorns with the bottom of a heavy skillet, or in a mortar and pestle. Transfer spices to a small bowl; add 1/4 cup salt and sugar. Rub mixture all over shoulder, including in the slits, if skin is on. Place in a large glass baking dish, cover, and refrigerate for at least 6 hours or preferably overnight.
Preheat to 425°F
Take pork out of the refrigerator 1 1/2 hours before cooking and let it come to room temperature. If skin is on, wipe the sugar/salt off with a damp paper towel as best you can (so it won’t burn). If no skin, just brush off any extra salt or spices clinging to the meat.
Place the pork, skin (or fat)-side up, in a roasting tray and pop in the preheated oven. Roast for 30 minutes until the skin puffs a bit (or the fat browns).
At this point, turn the heat down to 325°F
Add 1 cup boiling water to pan and cover the pork snugly with a double layer of tinfoil, pop back in the oven and roast for a further 5 hours.
Take the foil off and check the roast. It should be very tender. If the skin needs further crisping, turn the heat up to 500°F and let it brown for 5-10 minutes but watch it carefully so it doesn’t burn!
Transfer pork to a rimmed baking sheet; set aside. Strain the fat from the liquid and and pour the de-fatted juices into a small saucepan. Stir in 1/4 cup vinegar and brown sugar and bring liquid to a boil. Cook, occasionally spooning off fat from surface, until a thick, syrupy glaze forms, about 10 minutes (there will be about 1 1/2 cups glaze). Season glaze with salt and pepper as needed; set aside and use as gravy.