Prep the crockpot: Lightly spray the insert with cooking spray for easier cleanup. Add the sliced onions to the bottom.
Season the chicken: Sprinkle both sides of the chicken with salt, pepper, smoked paprika, and chili powder.
Place the chicken on top of the onions.
Mix the sauce: In a bowl, stir together BBQ sauce, chicken broth, garlic, brown sugar, and apple cider vinegar. Taste and adjust sweetness or tang if needed.
Pour and cook: Pour the sauce over the chicken. Cover and cook on Low for 4–6 hours or High for 2.5–3.5 hours, until the chicken shreds easily with two forks.
Shred and thicken: Transfer chicken to a board and shred.
Return it to the crockpot and stir into the sauce. If the mixture looks thin, leave the lid off and cook on High for 15–20 minutes to thicken.
Taste and finish: Add more BBQ sauce if you want it saucier, or a splash of vinegar for extra tang. Adjust salt and pepper.
Serve: Pile onto toasted buns with coleslaw, pickles, or sliced jalapeños.
It’s also great over rice, baked potatoes, or in tacos.