Place the onions in even layer in the slow cooker. Combine the spices in a small bowl. Rub the spice mixture all over the pork so it's well coated add to the slow cooker. Add Dr Pepper. Cover and cook on low for 7-8 hours or high for 3-4 hours.
Once done, turn off the slow cooker and remove the pork to a cutting board. Shred with two forks. Pour the leftover stock and onion mixture through a strainer into a measuring cup or bowl so you save the liquid.
Return the pork and onion mixture to the slow cooker and mix together. Add some of the stock to keep moist and stir in BBQ sauce to taste.
You can replace Dr Pepper with some stock and vinegar or your favorite beer.
You can sear in a Dutch oven and then simmer for 3-4 hours if you want to skip the slow cooker.
If the pork is still tough, that means it isn't done so just keep in the slow cooker long.
Leftovers will last for 3-4 days in the fridge or 3 months in the freezer.
To reheat pork, place back in the slow cooker on warm for 2 hours or in a saucepan over medium heat for a few minutes. You can also microwave.