Blend tomato paste, chipotle in adobo, and water, and set aside.

Dice onions and spread over bottom of slow cooker. Add about half the sauce mixture.

Take the pork shoulder. Using a sharp knife, slash through the rind and layer of fat and well down into the meat in roughly a diamond pattern.

Place the meat in the slow cooker. Take the rest of the sauce mixture and work as much of it as possible down into the slashes to that it reaches and seasons the meat. Spread the rest around the outside of the meat.

Cover and cook on Low heat for 8-10 hours.

Remove the meat from the pot. Remove and discard rind and fat layer, and bone. Pull meat apart and return to the sauce in the pot.

Cook another 2-6 hours on low. (You can skip this step if pressed for time - but it does improve the final result.)