Put fennel seed, coriander seed, and peppercorns in a small heavy skillet over moderate heat. Toss seeds frequently to evenly toast. When the fennel seed turns light brown, turn on exhaust fan and add red pepper flakes - toss several times and remove from heat. Place on plate to cool.
Pour cooled spices into a coffee grinder or blender, and add chili powder, salt, and cinnamon. Blend until spices are finely ground. Keeps in airtight container for 4 months, or freeze up to one year.Heat 2 TBS olive oil in large skillet over moderate heat. Add the garlic and cook until the cloves start to color. Add the sage and let the leaves sizzle for 10 seconds, then add sliced onions and salt and cook for 2 minutes. Add the water, cover, reduce the heat to low, and cook until onions are quite soft (about 15 minutes). Uncover and cook until most of liquid evaporates. Do not allow onions to color. Let cool to room temperature.
With a boning knife, carefully cut between the muscles of the pork to open it so that it lies flat like a book. Rub the surface with 1 TBS of the spice rub; spread cooled onions on top.
Reshape the pork to enclose the onions and tie lightly with kitchen string. Rub the meat all over with oil, then with the remaining 3 TBS of spice rub (you can refrigerate the pork overnight at this point, but bring it back to room temperature before continuing).
Preheat the oven to 275°. Line a roasting pan or heavy rimmed baking sheet with heavy-duty foil. Put the carrots, celery, and onion halves in the roasting pan to form a rack for the pork. Place pork on veggies and roast until fork-tender - about 7 hours. Let cool before removing string. Discard veggies and transfer pork to platter. To serve, pull the meat into chunks rather than carving. Serves 8-10.