Place onion and garlic in an even layer in the bottom of the slow cooker and pour in broth. Combine the salt, brown sugar, paprika, chili powder, garlic powder, mustard powder, cumin, and pepper in a small bowl. Rub the entire amount of spice mixture over the pork and place on top of the onions in the slow cooker.

Cover and cook until the pork is fall-apart tender—about 6–7 hours on high or 8–10 hours on low.

Turn off the slow cooker and use a slotted spoon or spatula to remove the pork and place on a cutting board. Use two forks to shred the meat, discarding excess fat and any undesirable parts. Place pulled pork into a large bowl and set aside.

Strain the liquid from the slow cooker into a separate bowl. (If you love onions, you can take them from the strainer and add them in with the pulled pork.) Skim as much oil from the top of cooking liquid as possible then pour back into slow cooker, along with pulled pork. Set on low or warm then serve when ready.