PORK: Heat the olive oil in a large saute pan over medium heat until hot. Add the onions and a pinch of salt and pepper. Reduce the heat to medium-low and cook for about 1 minute. Add the sage and cook until the onions stop releasing water (about 3 minutes). Add the water, cover and cook until the onions are very tender (about 10 minutes). Uncover and saute until the onions are very soft and the pan is dry again (about 2 minutes). Season well with salt and pepper.
Preheat the oven to 275 degrees F. Peel back the pork skin and spread the onions directly on the fat layer. Fold the skin back over the onions and tie closed with kitchen string. Season well all over with the Fennel spice. Arrange the meat on a rack in a roasting pan and cook until the meat is very tender (approx. 6-8 hours). The meat is ready when it pulls away easily if picked at with a pair of tongs. It is often easiest to cook the meat overnight or put it in the oven in the morning and let it cook all day - it doesn't need to be attended.
FENNEL SPICE RUB: Put the fennel seeds, coriander seeds and peppercorns in a heavy pan over medium heat. Watch carefully, tossing frequently so the seeds toast evenly. When light-brown and fragrant, pour the seeds onto a plate to cool. They must be cool before grinding or they will gum up the blades. Pour the seeds into a blender or spice grinder and add the salt. Blend to a fine powder, shaking the blender or grinder occasionally to redistribute the seeds. Store in a tightly-sealed glass jar in a cool, dry place, or freeze.