Pat down your pork shoulder with a few paper towels. Place in the slow cooker insert.
Combine all the spices in a small bowl. Make sure to mix well! Pour about half your spice mix into another bowl and set aside.
Take half of your spice mix and rub all over your pork shoulder. Really rub it in! Make sure to get all sides.
If you feel like you need more spice mix - and you might! - sprinkle a little more into your other spice bowl. The key here is not to touch the remaining mix. Don't contaminate it! You might have some left over that you can save for later.
Once your pork is thoroughly covered, make sure the fat cap is on top. If you're using sliced onions, place them under the pork now! Cover and cook on low for at least 10 hours.
You'll know your meat is ready when you can easily pierce and shred with a fork. Hooray!
Remove the pork from the slow cooker and place in a large bowl to shred. Remove onions, making sure to drain them, and set aside in a small bowl.
Carefully remove the fat cap and set aside. Shred however you see fit, removing fat as you see it. While shredding the meat preheat your broiler and line a baking sheet with foil sprayed veryyyyy lightly with cooking spray. If you're making bbq sliders, go ahead and toss the meat in your sauce now. (I LOVE Trader Joe's Carolina BBQ Sauce for this.) Place your shredded meat on the lined baking tray, ensuring it's a nice even layer.
Broil until dark brown and crisping on the edges, stirring every few minutes or so to prevent sticking.
Plate and serve!