Mix the chili, garlic and onion powders, salt, dry mustard, paprika and ground fennel and celery seeds together with a whisk in a small bowl. Rub the pork shoulder all over and press dry rub into the meat. Set aside.
Chop an onion and place in the bottom of a slow cooker. Place the dry rubbed pork shoulder on top of the onions and pour 1 bottle of root beer and your favorite BBQ sauce over the pork. Cover and cook on Low for 8 hours.
Pour 1 bottle of root beer in a small saucepan. Bring to a boil and turn to medium-low and simmer for 15-20 minutes until reduced to about 1/3 cup of syrup.
Remove cooked pork to a plate, remove any bone and pull the pork (shred). Set aside.
Pour the onion, root beer and BBQ sauce mixture from slow cooker into the saucepan with the root beer reduction and either use a hand immersion blender or pour it into a blender and purée. Cook and reduce until it is a catsup like consistency and pour over the pulled pork.
Serve the glazed pulled pork on a roll or bun of choice with coleslaw.
Video
Notes
Add the chili, garlic and onion powders, salt, dry mustard, paprika and fennel and celery seeds to a spice grinder or food processor and pulse several times to blend if you only have whole fennel seeds to start with. I keep ground fennel seeds on hand in a small airtight container for different spice blends.