Slice onions and place them in the bottom of the slow cooker.
Sprinkle onions with salt, pepper, and 1 tbsp BBQ rub.
Place brisket on top, fat side up.
Season brisket with salt, pepper, and remaining 1 tbsp BBQ rub.
Pour 1 cup BBQ sauce all over the top.
Cover and cook on LOW for 6 hours, or until tender and shreddable.
Slice or shred, spoon sauce over the top, and serve for the holidays.
