Sprinkle brisket with salt and pepper. Place brisket in 6 or 7-quart slow cooker; top with onions.
In small bowl, whisk beef broth, ketchup, chile powder, paprika, and cumin; pour over onions.
Cover and cook on low 10-12 hours until brisket is very tender.
Remove from slow cooker and let stand 15 minutes before slicing.
Shred and reserve 4 cups brisket for brisket and black bean tacos recipe.
Skim fat from cooking liquid. Serve remaining gravy with brisket.