Place chicken in 5 - 7 quart slow cooker (I use this one).
In a small mixing bowl whisk together chili powder, paprika, garlic powder, cayenne pepper, salt, and pepper.
Pour spice mixture over chicken and add grated onion. Toss well to evenly coat chicken with spices.
Evenly pour 3/4 cup BBQ sauce over the chicken.
Cover and cook on low heat 4 - 6 hours until chicken shreds easily and registers 165 degrees in center.
Transfer chicken to a cutting board, reserve liquid from the slow cooker. Skim fat from the liquid and discard the fat.
Shred chicken with two forks (I do it a little larger than I want it to end up because as you toss again in the next step it shreds up a bit on it's own).
Return chicken to slow cooker. Heat remaining 3/4 cup BBQ sauce with 2 Tbsp steak sauce in a microwave safe bowl in the microwave until warm (about 1 - 2 minutes).
Pour 3/4 cup of the reserved liquid and the BBQ sauce mixture over the chicken then toss to evenly coat. If you feel it needs a little more moisture you can add another splash of the reserved liquid (or more BBQ sauce), also season to taste with a little more salt if needed.
Serve the pulled bbq chicken in hamburger buns (I also like to add coleslaw, with a little blue cheese mixed in sometimes too).