Layer the base: Place sliced onion and minced garlic in the bottom of the crockpot.
Mix the sauce: In a bowl, whisk BBQ sauce, chicken broth, brown sugar, apple cider vinegar, smoked paprika, chili powder, black pepper, salt, and Dijon mustard until smooth.
Add the chicken: Place chicken on top of the onions.
Pour the sauce mixture over the chicken, coating it well.
Slow cook: Cover and cook on low for 5–6 hours or high for 2.5–3.5 hours, until the chicken is very tender and shreds easily.
Shred the chicken: Remove chicken to a cutting board. Shred with two forks, then return it to the slow cooker and stir to coat in sauce.
Simmer to thicken (optional): Leave the lid slightly ajar for 10–15 minutes on high, stirring occasionally, to reduce and thicken the sauce if it seems too thin.
Toast the buns: Lightly butter and toast buns in a skillet or under the broiler for 1–2 minutes until golden. This helps them hold up to the saucy filling.
Assemble: Pile the BBQ chicken onto buns.
Add toppings like coleslaw for crunch, pickles for tang, or cheese for extra richness.
Serve and keep warm: Switch the crockpot to “warm” for party service. Stir occasionally to keep everything evenly sauced.