Add the sliced onions and garlic to the bottom of a slow cooker and pour in chicken stock.
In a small bowl, mix together the spice mix until evenly combined.
Pat chicken breast with paper towels to remove any excess moisture from packaging.
Generously sprinkle both sides of chicken evenly with spice mixture (you may have some left over). Gently press spice mixture onto chicken to help it adhere.
Lay chicken, in a single layer, over onions. Cover the crockpot and cook on high 4-6 hours or 8-10 on low, or until chicken is tender. Remove the chicken to a plate and set aside.
Set a sieve over a heat-proof mixing bowl and pour the onion-liquid mixture into the sieve. Place the drained onion mixture on a cutting board and roughly chop them into bite-sized pieces. Return the chopped onions to crockpot and discard the liquids.
Using 2 forks shred the chicken breasts into bite-sized pieces. Return the shredded chicken to the crockpot and stir to combine with the onion mixture.
Add prepared BBQ sauce and combine to chicken mixture. Cover the crockpot, turn on high, and heat just until warm through. Serve.