Prepare the pork:

1. Season the pork with salt and pepper, then leave it aside while you assemble the rest of the ingredients.
2. In a large skillet, heat the oil over medium heat. Cook, stirring periodically, until the onions and garlic are delicious and browned around the edges. Combine the tomato paste, chili powder, and cumin in a mixing bowl. Cook, stirring constantly, until the color of the tomato paste changes from brilliant red to orange, about 3 minutes. Cook for 1 minute, scraping up the browned bits from the bottom of the pan with the water, apple cider vinegar, and fish sauce.

Cook the pork:

1. In the bottom of a slow cooker, place the seasoned pork, then pour the onion and spice combination on top. Wiggle the pork around in the slow cooker’s bottom, allowing some of the onion and spice combination to fall underneath. Cover and simmer on LOW for 6 to 8 hours or HIGH for 4 to 6 hours, or until the pork is easily shredded.

To finish up:

1. Remove the pork from the slow cooker and place it on a chopping board. Remove any bones from the pork and discard them. Shred the meat with two forks, removing any connective tissue and large clumps of fat.
2. Skim the majority of the fat from the surface of the cooking liquid. Remove the fat and set aside the cooking liquid.
3. If using barbecue sauce, pour it into the slow cooker with the shredded pork and stir to combine. Return part of the reserved cooking liquid for even additional flavor. If you’re not using barbecue sauce, return the shredded pork to the slow cooker along with the cooking liquid. Season to taste with salt, vinegar, or spicy sauce, if desired.