Brown the chopped onion and garlic in a heavy pot in the oil.
Add the roast and brown on all sides.
Add the water or broth and cover the pot.
Simmer on medium low for 3 to 6 hours, making sure that the liquid doesn't evaporate, until the bone can be removed easily and the meat falls apart.
Pour off the cooking liquid, leaving about 1/2 cup.
With 2 forks, shred the meat until it is no longer chunky.
Remove the outer skins from the tomatillos.
In a sauce pan of boiling water, cook the tomatillos and jalapeño pepper for 5-7 minutes, until they change from bright green to yellowish green.
Cut 1 inch from the bottom of the pepper and remove any seeds that might be inside.
Place the cooked tomatillos and the piece of pepper into the blender with crushed garlic and salt.
Puree until smooth.
Pour the sauce over the shredded pork and gently mix until evenly distributed.
Serve with fresh salsa and/or fresh cilantro.