Rub the pork shoulder all over with salt and pepper, pressing into every crevice. Place into the slow cooker and let rest 20 minutes.

Heat oil in a wide skillet over medium heat. Add onion and garlic, cooking and stirring until sweet and browned around the edges, about 5 to 7 minutes.

Stir in the tomato paste, chili powder, and cumin. Cook, stirring, until the tomato paste turns from bright red to orange, about 3 minutes.

Add the water, apple cider vinegar, and fish sauce. Cook for about 1 minute, scraping up any browned bits from the bottom of the pan.

Pour the onion and spice mixture over the pork. Move the pork around so some of the mixture slides underneath it.

Cover with the lid and cook until very tender: LOW for 6 to 8 hours or HIGH for 4 to 6 hours.

Transfer the pork to a cutting board. Remove and discard any bone. Shred the meat using two forks, discarding any connective tissue and large clumps of fat.

Pour the cooking liquid into a container and skim most of the fat from the surface. Discard the fat and set the liquid aside.

If adding BBQ sauce, mix it into the shredded pork with a splash of the reserved cooking liquid. Otherwise, return pork and cooking liquid to the cooker. Taste and season with salt, vinegar, or hot sauce as needed. Serve immediately.