Arrange a rack in the middle of the oven and heat to 500°F. (Use convection if you have it.) Line a rimmed baking sheet with aluminum foil, then fit a wire rack inside it.Place the salt, brown sugar, paprika, black pepper, chili powder, smoked paprika, dry mustard, garlic powder, onion powder, cornstarch, cayenne, and celery seeds in a small bowl and stir to combine.Pat the pork dry with paper towels. Evenly coat the pork with 1/2 of the rub, then place it on the wire rack fat-side up if possible. Roast until the pork is sizzling with a bit of char on the edges, about 10 minutes, but don't let the spices burn.Meanwhile, place the onions in a 6-quart or larger slow cooker. Sprinkle with the remaining rub. Pour in the cola, vinegar, and Worcestershire.Transfer the pork to the slow cooker. Cover and cook on the LOW setting until the pork is soft enough to pull apart with a spoon, maybe 8-10 hours? I would check around 6 for tenderness and go from there. Transfer the pork to a large bowl and let stand until cool enough to handle. Pull the meat apart into large chunks or shred with a fork, discarding any bone, connective tissue, and large clumps of fat.Fit a fine-mesh strainer over a medium bowl. Pour the cooking liquid through the strainer and discard the solids. If you are serving immediately, spoon off as much fat as possible from the surface. For best results, refrigerate until the fat solidifies and collects on top, then scrape off and discard the fat (keep the meat covered and refrigerated during this time).Toss the meat with enough defatted cooking liquid to moisten. Serve the meat warm, or let cool, cover, and refrigerate to use in other recipes and with your favorite BBQ sauce.
For the Slaw: Shred your cabbage and carrots or purchase a package of Coleslaw from the supermarket. Combine the dressing ingredients and check for seasonings before pouring on your cabbage. If you want sweeter, add a little sugar. I like sweet coleslaw. Like it vinegary, add some vinegar. When you get your dressing right, pour on your slaw and combine.
For the Sandwich: Butter and toast your buns lightly on each side. Put a little BBQ on the bottom bun and pile some pulled pork on. Pour on a little more BBQ sauce, top with your coleslaw and place the lid on top! You will need a lot of napkins! Enjoy!